Self Join for groups refers to whether or not users are allowed to join a group by themselves (when Self Join is "Yes") or if a user has to be manually added to a group by someone with the Manage Groups permission to be able to join a group (when Self Join is "No"). Setting Self Join to "Yes" or "No" depends on the purpose of the group as well as the permissions granted to the group. Usually groups that have higher-risk permissions will have Self Join set to "No", while groups that users are generally free to join and leave as they please will have Self Join set to "Yes".
We strongly recommend setting Self Join to "No" for any group that has user permissions, or equipment checkout privileges associated with it.