[development] Assign one role to a user

Hello Guys, I am planning to allow some users to add users to the site. The only concern is I want them to select only one user role, so, instead of a checkboxes, i want a select list or radio buttons. It can be achieved via form alter, but just wanted to know if there is any module available for this I am on Drupal 6. -- Regards, Vaibhav Jain