You could have same key for
everyone, granulated by group for instance.
If you really want to do it in that fashion, take a look at AuthorizedKeysFiles and Match directive for your SSH server.
It should be doable, in a way when user is created and belonging to a group and has public key to login without additional intervention, since the key is effectively per group configured globally on ssh server.
I have not tested this scenario, nor used it and hopefully i understood the requirement.
Spin up couple of vm boxes and give it a shot
Regards
Peasant.