An agency roster is the list of freelancers who have joined an Upwork agency on the platform, each with a role and an exclusive or non-exclusive status, from which members are assigned to the agency's contracts.
The roster is managed by the owner and any admin. Members are added by invitation, which the freelancer accepts from their own account, and removed by either side; each entry records whether the member is exclusive to the agency and what permissions they hold, from a plain member who can only work on assigned contracts up to business manager or admin. Only people on the roster can be assigned to an agency contract, so a developer the agency employs but has not added cannot bill through it. Part of the roster is public: the agency profile shows a members section with the profiles the owner chooses to display, and a client can open each one.
The roster and the declared team size are different things, and the difference is informative. Team size is a bracket the owner types in; the roster is the set of accounts actually linked. An agency declaring 51 to 200 people with three profiles in its members section is either a large firm that keeps most staff off Upwork, which is common, or a small one with ambitions.
Upwork's Help Center sets a limit on how many agencies a freelancer can join and on the roles within one; both are Upwork's to change, and the roster is where they apply.
How Upwork Insights uses it
This term describes Upwork's own rules or product rather than a figure the index measures; the pages that do carry live figures are the stats and rate pages.