Federated Services
These are my number 1 go-to for folks looking to set up their own communities since they generally allow people to set up their own servers for their own groups and allows their independent groups to communicate with other hubs using the ActivityPub protocol. Think of the way how email works - you can get an email address from google, yahoo, microsoft, but with that address you can send email to anyone else who is also has an email address, even with a different provider.
All of these options are open-source and free software!
You can find more info on available hubs to join using the site: https://fediverse.network/