The closest platform that is a somewhat known group of people coming together in a semi-private space seems to be Discord. You can have as large or as small a

The closest platform that is a somewhat known group of people coming together in a semi-private space seems to be Discord. You can have as large or as small a community you want, you can have known people, unknown people. Mastodon comes close as well, but the effort to start a discord community is so much smaller compared to running a Mastodon instance.