Even if we provided the ability to create user accounts in advance, new users would still need to set a password. So an invite email would really only serve to inform them about the existence of the forum.
That might still be useful though, especially if users who aren't the forum owner can play a part in inviting new users.
Plus it's relatively accepted that creating some artificial scarcity can be good ("this forum is invite only, here's your invite"). If you want to go that route.
I don't see any implementation problems apart from the obvious rate limiting / spam protection.
Even if we provided the ability to create user accounts in advance, new users would still need to set a password. So an invite email would really only serve to inform them about the existence of the forum.
That might still be useful though, especially if users who aren't the forum owner can play a part in inviting new users.
Plus it's relatively accepted that creating some artificial scarcity can be good ("this forum is invite only, here's your invite"). If you want to go that route.
I don't see any implementation problems apart from the obvious rate limiting / spam protection.