A friend recently moved to a rural area, and this question came up. I mentioned Fediverse as one possibility, with caveats.

I think you'd have to go through and make sure you know exactly what to tell them to install/configure for each of the following scenarios:

* Willing to install an iOS app.

* Willing to install an Android app.

* Willing to create an account in a Web site, and want to get email when someone in the group says something (with a hashtag or whatever).

Also see whether they're willing to talk on Fediverse, or they want something less public.