Ask for suggestion

Hi.
I need suggestions from you. I am starting this system that has just one database and several types of users accessing different parts: tables, views, etc.
The issue I have is every type of user has a different way to register into the system and a different layout for their username. Some of them register themselves some other need to be approved after and the data for the registration is different as well. The admins are preregistered.
I think I can have the whole system for the admins set up and have different systems just for each of the users but this can be a problem in case of modifications somehow.
Any suggestions are welcome.
Regards