Hi
I think there could possibly be an error with this line:
Roles.AddUserToRole("UserName", "RoleName");
Looking at some code examples, I found the following for illustration, it will need modifying for your code:
Roles.AddUserToRole(Username.Text, RoleList.SelectedItem.Value)
Maybe yours should be:
Roles.AddUserToRole(UserName.Text, RoleName.Text);
The point I am trying to make is that it might be that the way you have it at the moment, it is not reading the Text property of the UserName properly. So say you try and create a UserName of Morgan, with your existing code this is read as Username, but if you used "Username.Text" instead it would be correctly read as Morgan.
Rgds
Accelerator