For anyone who wants an invitation to join mastodon.online, let me know! Like mastodon.social, it is one of the two official communities run by the Mastodon non-profit. The difference is that mastodon.online is invite-only.
Joining an invite-only server helps keep out spammers who target mastodon.social and lets someone be part of a more exclusive community. This can help someone have a better-quality mastodon experience, overall.
Of course, to keep out scammers, the invitation isn’t open to just anyone who asks, they will have to have a posting history of some sort.
Invitation-only Mastodon instances prevent spammers by controlling who can join and maintaining a trust-based network. Since every new user must be invited by an existing member, admins can track invitation chains. If someone turns out to be a bad actor, admins can ban them and also ban anyone they invited. This is like pruning the whole spam network. This discourages spammers because their accounts can be removed along with their entire invitation tree.