When you have got scenario where in, a user is belonging to multiple roles and you want him/her to select his choice of role while login to portal.
This situation can be solved by developing a custom login portlet and make use of Authentication Filters.
Here is how this would work,
User login using your custom login portlet with his userid & password.
Once user submit, control goes to authentication filter and there you connect to your user repository to fetch user roles and preset a screen ( next screen of login portlet ) to select his roles.
When user selects his roles and click Login, control again goes to authentication filter and there you authentication user and redirect user to portal landing page.
I hope that helps.Feel free to post your queries here.