@gnu2 @administrator
Normally, people outside one's instance cannot join a group created on that very instance. For reasons I don't understand I was able to join two groups on nu.federati.net which runs Gnusocial version 2.0.0-dev, an earlier version than gnusocial.net's version 2.0.1-beta0. People on an instance from the Mastodon-network told me they cannot join groups on gnusocial.net.