An agency member is a freelancer who has accepted an invitation to join an Upwork agency and can be assigned to the agency's contracts, with the work billed under the agency's name and paid into the agency's account.
Membership starts with an invitation from the owner or an admin, accepted from the freelancer's own account, and it leaves the freelancer's individual profile intact. What changes is who can contract with them. When a client hires the agency for a job, a member is assigned to the agency contract; the hours or milestones are billed by the agency, the client pays the agency, and the member is paid by the agency outside Upwork's system, on whatever terms the two have agreed. Upwork's Help Center is explicit that the agency, not Upwork, is responsible for paying its members.
Members come in two kinds. An exclusive member works on Upwork only through the agency; a non-exclusive member keeps taking independent contracts too. The choice is made per member and shows on the freelancer's profile, where clients can see the agency affiliation.
Contracts worked through an agency feed the agency's Job Success Score and earnings total, and Upwork's Help Center explains how the same contracts are reflected on the member's own profile; the two records are kept separately. Note also that the team size an agency shows is a bracket it declares, not a count of its members, so an agency with three members may say it has fifty people.
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.