Membership of the company is strictly limited to those who have been duly appointed by the parochial (or guild) church council to hold the office of parish clerk in certain ancient parishes in the City of London and its immediate suburbs.

At the time of the charter of 1639, under which the Company now acts, these consisted of 97 parishes within the walls, 16 parishes in the liberties (including four in the Borough of Southwark) and one (St. Margaret's) in the City of Westminster and 15 out-parishes adjacent.  21 'historical' parishes were added to the Bills of Mortality within this catchment area up to 1824 and the parish clerks of these are therefore entitled to apply for membership.  In this way the number of Brethren is limited to 150.

Candidates for admission, who are required to produce a certificate of their appointment as parish Clerks, must be proposed and seconded by member's of the Company, and either the proposer or the seconder must be a member of the Court of Assistants.

It should be noted that a candidate must have been known to his proposer for at least one year before an application for membership will be considered by the Court.