Works in free edition!
mattermost=# select token, extra from tokens where type = 'team_invitation';
token | extra
------------------------------------------------------------------+----------------------------------------------------------------
bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb | {"email":"test@some.email","teamId":"aaaaaaaaaaaaaaaaaaaa"}
Per /server/channels/app/email/email.go:470
https://yourmattermost.com/signup_user_complete/?md=email&t=bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb&d={"email":"test@some.email","teamId":"aaaaaaaaaaaaaaaaaaaa"}