Logging in, choosing a role and not letting anyone else choose that role.

I'm pretty new to asp.net, and didn't know where else to put this, so let's hope here is okay.
I'm building a system which will run during a set time period. During this time period, users can log on to the system, choose a role they wish to play in the next part of the application and then proceed. Each role can be chosen only once, so there will only be one user in each role (there's about 12 roles, selected from a database, so the roles aren't permanently fixed). In the next part of the system the roles will be sharing data and interacting with each other in different way, but first I'm concentrating on the role selection procedure.

There's a few issues here to resolve, and I'm unsure of the best way to go about it. Firstly, I need to prevent users selecting roles that are already selected. My first instinct is to store a list of which roles are selected in the database, and if the user selects a role that is marked in the dat ...