@Elissa I'm afraid Groups is the one crucial functionality Fediverse is missing at the moment. 😿 Friendica and Hubzilla have "forums" that work best inside these federating parts (so not with Mastodon). AFAIK their forums are public. Smithereen has groups, but it's a new Fediverse project in alpha/beta stage, its groups work only within itself. For Mastodon there is a Guppe "addon" - https://gup.pe - but again it's public posts. I hope private groups will appear (some day?)
This is a brand new server run by the main developers of the project as a spin-off of mastodon.social It is not focused on any particular niche interest - everyone is welcome as long as you follow our code of conduct!