As far as I can tell they’re all free. I don’t the service myself, though. You can (should) add multiple relays to whatever app you use to use the protocol to the fullest.
The only account you have is the key and metadata stored on your device is my understanding. You can use any account with any relay and create as many as you need. All you need to sign in is your private key. Enter that into any nostr app connected to any nostr relay(s) and you’re good to go.
They don’t do communities last time I checked, it’s more of a Twitter alternative. The “shouting into the void” social media lends itself much better to moderation free environments than any attempts at discussion forums.