I thought that setting "Allow anyone to sign-up from login page" and "Include this team in the Team Directory" to "yes" would allow people to join my team without invitation, but that doesn't happen. When a user that were not invited try to join a public team via Gitlab SSO the following error appears:
Mattermost needs your help:
We couldn't find an existing account matching your authentication type for this team. This team may require an invite from the team owner to join.
The log shows pretty much the same (by the way, "EROR"?):
[2016/02/15 08:28:35 BRST] [EROR] /signup/gitlab/complete:SqlUserStore.GetByAuth code=500 rid=xxxxxx uid= ip=xxxxxx We couldn't find an existing account matching your authentication type for this team. This team may require an invite from the team owner to join. [details: teamId=xxxxxx, authData=xxxxxx, authService=gitlab, sql: no rows in result set]
Joining teams with invitation works nicely, and if the user creates a new account through mattermost he can join the public team without invitation as expected.
I have a 'standalone' installation of Mattermost 1.4.0 with Gitlab SSO enabled (I'm not using the Gitlab omnibus package).