Your solution will completely depend on whether you want to use Mendix' user roles to achieve this functionality. If so, you should make a page where users can change their userroles themselves. Check out the Account_New or Account_Edit page for an example on how this is setup for administrators. Keep in mind that this might create a security risk,
Another option is to create similar functionality yourself by creating a new entity and setting XPath constraints on whatever roles are connected to the user's account.