[otrs] Customer groups

Hello, I don't know if I explained this well, so let's try it again :-( I need that the customer can choose from a list in the web interface, the group he belongs to. I've a newbie with OTRS and although I'm trying to learn how it's works as quickly as I can, but I don't know how I can get this. Maybe with different creating account pages for the users (the users groups are defined and won't change) could be easier, but I'd need to know the way to tell OTRS that the user must be created for a specific group. I'd thank any idea or clue about how I could get this or what I should to read to be able to program that. Thank you very much in advance, J