The two could be integrated I think. You could use nip 86 (link please?) to sell admission to the group. A bot with the admin key could then grant or revoke access based on subscription status.
Discussion
Oh, that's essentially what you said 🤦
https://github.com/nostr-protocol/nips/commit/a9f7058659171d8f4a0f788751a39ad78b95f557
I guess this isn't nip-86 yet but it looks like it would work for paywalled content. people without the shared key would see encrypted data or nothing at all... better would be to make it so they could see how to decrypt it