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"}