Two reasons mainly, firstly that only some of the users should be allowed to use the DNN site, & secondly that my ASP.Net skills are a little limited.

Is there some DNN API that I can call to add the user to the role? I see there's a Stored Procedure (AddUserRole) that adds a user to a role, but it requires the UserID (int) from DNN's Users table, which is not the same as the UserID (GUID) from ASP.Net's aspnet_Users table. So I guess to add the user to the role, I need to look them up in DNN's Users table using their username, which will give me their UserID, which I can then use to call the AddUserRole Stored Procedure.