Any relay can be used to host a Flotilla community. The same content and access policies implemented by the host relay are automatically applied to group members. Flotilla auto-detects these limits by probing relays on users' behalf.

If you're interested in setting up a relay with custom policy, I recommend https://github.com/coracle-social/triflector, which supports relay invites and different AUTH policies (including a custom backend). If you're less technical and don't want to self-host, take a look at relay.tools — nprofile1qyghwumn8ghj7mn0wd68ytnhd9hx2tcpzfmhxue69uhkummnw3eryvfwvdhk6tcppemhxue69uhkummn9ekx7mp0qyfhwumn8ghj7am0wsh82arcduhx7mn99uq3vamnwvaz7tmzv4mx7tnwdaehgu339e3k7mf0qydhwumn8ghj7argv4nx7un9wd6zumn0wd68yvfwvdhk6tcqyp7vx29q3hdj4l0elxl800hlfjp538le09epsf7k9zj59ue2y37qurt2875 is working hard to make it easy for people to implement relays with custom policy. Not that hosted relays don't improve nostr's decentralization, but they can be useful for low-stakes use cases.