You should be able to do this with a free module (such as the REPORTS module).  For instance, using the free Advanced Datagrid module I've blogged about you can get what you want with the following query:
 
 select distinct u.username, r.rolename
 from users u
 inner join userroles ur on u.userid = ur.userid
 inner join userportals up on u.userid = up.userid
 inner join roles r on r.roleid = ur.roleid
 where up.portalid = [dnn:PortalID]
 and ur.roleid in
 (select ur1.roleid
  from userroles ur1
  where ur1.userid = [dnn:UserID])
 and (r.rolename != 'Registered Users')
 and (r.rolename != 'Subscribers')
 
  
Note the final two lines that filter out the results so that the "Registered Users" and "Subscribers" groups are not considered.  If you included the "Registered Users" group then all users with an account on the portal would always be listed.  Since "Subscribers" is usually auto-added to new accounts by default you'll want to filter that out as well unless you've changed your portal's default settings for the Subscribers group.
 
Final Note
Once you've gotten things working the way you like you should move your query into the database as a stored procedure.  Also note that this query as written is not very efficient -- it should work well for most people but could start to bog down if you have more than a few thousand accounts on your DNN installation.
 
Cheers!
-mamlin