nostr:nprofile1qqsf03c2gsmx5ef4c9zmxvlew04gdh7u94afnknp33qvv3c94kvwxgsppemhxue69uhkummn9ekx7mp0qy2hwumn8ghj7un9d3shjtnyv9kh2uewd9hj7qgewaehxw309a5xyu3wvdhhyctrd3jjuum0vd5kzmp0gankr4 is the nostr dev I know that's been thinking about private groups. Maybe he has some thoughts.

No one has built an end-to-end flow like this, but the pieces are slowly coming together. Flotilla supports invite codes, and making a zap receipt into an invite code would be trivial. The UI is probably the biggest lift, but some nip 29 apps might work well for this (I assume since it's a one-time event a single channel would be preferable to flotilla's many-room approach)

If you want strictly group chat that's kind of the the vision of https://flotilla.social/, but also works on https://chachi.chat/ today I think.

I would like something like that to also work in whatever app they already use too (i.e. relays that only some people write to and potentially only this same people read from), but it wouldn't be exactly like a closed group, more like a shared restricted feed, which is slightly different I think, such that it would fit better the microblogging model. Zapbox and Lockbox are some experiments to move us slightly closer to a world of weird custom relays that fit in automatically.

In any case most clients of today are not very close to enabling that. The exception is https://jumble.social/, but https://nostur.com/ and https://coracle.social/ are going there I think, and I heard https://damus.io/notedeck shares some of this vision and Amethyst is planning on joining the bandwagon too.