Founder members of the Royal Society (act. 1660–1663), may be defined as the group known as 'original fellows', which comprises those who joined the society between its foundation in 1660 and June 1663. The Royal Society is the oldest public institution devoted to the pursuit of scientific research, or, as recorded in its first minutes, 'the promoting of physico-mathematical experimental learning' (Birch, 1.3). From the outset it aspired to combine the role of research institute with that of a clearing house and repository of knowledge, while at the same time serving almost as a London club through its weekly meetings. The society has always been a voluntary body, and even in the seventeenth century it had quite a large membership, which peaked at 228 in 1669. Partly because of its finite nature, the membership has attracted various statistical analyses as a means of assessing the affiliations of science in this formative period—some of which have been more reliable than others.

The background to the Royal Society has been, and continues to be, controversial. Various earlier groups have been canvassed as its principal progenitor, including the group of natural philosophers who met at Oxford in the 1650s under the aegis first of John Wilkins and then of Robert Boyle, which was itself the successor to an earlier group which had met in London in the late 1640s. In fact, it seems likely that one of the society's strengths was the extent to which it drew on support from various quarters, and only those who were seen as a liability (such as the heterodox philosopher Thomas Hobbes) were discouraged from joining.

The society's inaugural meeting took place on 28 November 1660 in the rooms at Gresham College, London, of its professor of geometry, Lawrence Rooke, following a lecture at the college by Christopher Wren, professor of astronomy there. This was attended by twelve people, representing a variety of backgrounds. There were members or former members of the Oxford group: Wren himself, Robert Boyle, John Wilkins (in 1660 made dean of Ripon), and William Petty. Equally important were prominent members of the royal court: William Brouncker, Viscount Brouncker, who was to become chancellor to the queen and first president of the society, the Scottish politician Sir Robert Moray, Alexander Bruce, later second earl of Kincardine, and Sir Paul Neile. Last there were London physicians and intellectuals, including Jonathan Goddard, William Ball of the Inner Temple, Abraham Hill, and Rooke, host for the occasion. Another physician, William Croune, though absent, was nominated as 'register'.

The foundational twelve then drew up a list of the names of forty people who they thought should be invited to join; at the following meeting, on 5 December, it was also agreed that they should aim at a 'stated number' of fifty-five (Birch, 1.5). The men who appeared in the list of forty represented a variety of backgrounds: they included prominent virtuosi, often with court connections, like Sir Kenelm Digby, John Evelyn, and Elias Ashmole; other governmental figures with related interests, such as Sir Anthony Morgan and Thomas Povey; Oxford and Cambridge academics like John Wallis, Francis Glisson, and Seth Ward (from December 1661 dean, and from July 1662 bishop of Exeter); London medical men, such as George Ent (knighted in 1665), Christopher Merret, and Daniel Whistler; and a handful of more miscellaneous figures.

Over the next few months most of the men who appeared on that list were elected piecemeal in a sequence which can be deduced from the date at which they paid their admission money and became eligible to pay subscriptions. Already by the end of 1660 over thirty fellows had been elected—including some who had not appeared in the list of forty, such as the mathematician John Pell and the aristocrat William Cavendish, third earl of Devonshire—and in the early months of 1661 this process continued. The proposed ceiling of fifty-five had been passed before the beginning of March and by the end of the year nearly 100 fellows had been elected. By late May 1663 135 elections had occurred and it was those who had joined the society's ranks as a result who formed the category of original fellows. Thereafter the membership continued to grow with the seal of royal approval being given by the election of the king, the duke of York, and George Monck, earl of Albemarle, in January 1665. In June 1663 the society elected its first members domiciled abroad, the Dutch natural philosopher Christiaan Huygens (1629–1695) and the French savant Samuel Sorbière (1615–1670). A further seventy foreign members were elected by 1700, many of them attracted to the society by its unique commitment to experimental activity, and maintaining contact with it through correspondence.

At first sight the fellows may appear a unitary group—and so they were presented in the society's membership lists which were printed and circulated from 1663 onwards. However, they can be distinguished in various ways. From August 1661 we are frequently told the names of fellows' proposers, and this provides evidence of networks of patronage and acquaintance within the society. The society's full records make it possible to establish which fellows, once elected, provided the society with the support it needed, and which did not. From the outset officers kept account books detailing fellows' payment of subscriptions, and as a result it is known who provided the finance on which the society, as a voluntary organization, depended. In addition, the society's minutes recorded contributions to discussion at meetings, though attendance was only recorded in the minutes of the society's twenty-one man governing council. It is thus easy to deduce who played an active role in the society's business, and who dominated the society's proceedings. Moreover, from the time when its first charter was granted, in July 1662, the society had a president, council, treasurer, and two secretaries; various men served in these capacities over the subsequent years.

An analysis by Charles Webster of the most active fellows in the period 1660–63 was subsequently extended by Michael Hunter to the seventeenth century as a whole. From this it appears that the original twelve fellows were disproportionately active, not only in the society's earliest years but long afterwards. Various figures named in the list of forty putative fellows and elected soon after were also especially important, including John Evelyn, his fellow virtuoso Thomas Henshaw, and Christopher Merret. Others who were to play a significant role were elected in the society's earliest months although not named in the list, including Walter Charleton and the merchant Daniel Colwall. Another figure who came to prominence was the lawyer and virtuoso Sir John Hoskins, who was abroad travelling at the time of the inaugural meeting, and who was not admitted until December 1661. Also crucial were two men who were to become employees of the society: the first secretary, the German-born Henry Oldenburg (probably elected in December 1660), and Robert Hooke, who became a fellow only in June 1663, having been curator of experiments since November 1662. These men—and particularly the founding core—formed a group who imbued the society with an extraordinary energy, implementing a policy of corporate experiment and data-collecting that was to prove highly successful. It was also they who devised the institutional structure of the society which did much to ensure the body's extraordinary longevity: this was largely in place by the time of the second charter issued in April 1663, complemented by the statutes published in November that year.

Beyond this the society had the support of several dozen fellows who spoke at meetings, served on council, or paid their subscriptions on a regular basis. The latter were financially indispensable, and if rarely mentioned in the minutes may have been silent onlookers of the proceedings. The number of regular subscribers dwindled during the 1660s as the novelty of the institution wore off, while the proportion of members who made no contribution whatever increased, perhaps particularly in the aftermath of the king's election in 1665. During the 1670s and early 1680s this dead wood was removed by expulsion, though because many of those involved were aristocrats and courtiers whose names added an aura to the list the policy was pursued with caution. Gradually, however, the society came to see it as more crucial to record its real membership than this penumbra, and historians have come to appreciate that it is on analysis of the active rather than the nominal fellowship that assessment of the society's support must rest.

Of course, the society was socially selective. There were relatively few merchants among the fellows, and tradesmen were rarer still; no woman was to be elected until the twentieth century. Nevertheless, a wide range of professions and affiliations was represented, and equally wide was the range of fellows' interests. Early meetings heard Boyle speak on pneumatics, Goddard on colours, Bronneker on ballistics, Petty on clothmaking, Henshaw on saltpetre, and Evelyn on silviculture, and received reports on subjects ranging from the natural history of foreign countries to mining and magnetism. This variety, combined with the extraordinary energy of the core of men who set the society going and who were active in it throughout its early years, explains its success.