For accepting new users on a Mattermost system, there is no middle ground between "open system" and "invite only".

We were opening our Mattermost Team edition to interested people, and are now weeding out fake accounts manually. This could be prevented if we had an "approve new user" button for system admins.

Bonus: a configurable login page with custom text would give potential visitors a better idea about wether they should want to join this server. "You are about to sign up for the full-contact deep-sea chess entousiasts chat. Are you sure?"