Could dumb this down a step, remove subscriptions. Users pay nostr:npub18kzz4lkdtc5n729kvfunxuz287uvu9f64ywhjz43ra482t2y5sks0mx5sz to start up a private group. Grpstr owners share a URL to invite others?
Boom. Nostr private Groups?
Discussion
Private groups, generically speaking, should be free! Creatr is trying to serve a different need by helping creators monetize and access control notes and files behind a paywall. Of course nostr:npub13qrrw2h4z52m7jh0spefrwtysl4psfkfv6j4j672se5hkhvtyw7qu0almy if you are interested let me know, but I’m not sure that was your original goal.
I know Coracle recently rolled out a private groups solution but it is quite complex.