Put somebody on a team

By the address they signed up with. An address nobody has signed up with and confirmed yet is filed as an invitation instead: the row waits with userId empty and becomes a membership, with the permissions it was sent with, on the first request its owner makes once signed in. Adding an address that already holds an invitation here replaces it, so nobody is listed twice.

Either way the address is told by mail, and notified says whether that mail was accepted for delivery. A false there is not a failure of the request: the place is held, but nobody has been told yet, so send it again.