Max on Nostr: That's technically really difficult because a admin member has to do the commit to ...

That's technically really difficult because a admin member has to do the commit to add a new person to the group. You can however keep the app running in the background and do that automatically, or have dedicated small server admin that is on a 24-7 online machine.
I'm not sure if it's a good idea to have an optional rule where join commits can be made by members who are not even in the group.