Note: These operations are RESTful so that the APIs are easy to understand and access, and not bound to a particular implementation's model. Authentication is at the HTTP layer and not discussed here.
This document assumes JSON, however XML-based documents are easy to envision.
hasMember
GET /groups/groupid/members/memberid
Note: Because RESTful URLs are "eternal", groupid and memberid must be persistent (ie: pretty much non-changeable), and therefore should probably be implemented as UUIDs.
Returns: Member info or 404 if not found