Karlo,
For maximum flexibility and ease of use (ie, not having to write code), you should just use the below sql, put it into a query and save it:
PARAMETERS UserName Text;
SELECT DISTINCT MSysAccounts_1.Name
FROM (MSysAccounts INNER JOIN MSysGroups ON MSysAccounts.SID = MSysGroups.UserSID) INNER JOIN MSysAccounts AS MSysAccounts_1 ON MSysGroups.GroupSID = MSysAccounts_1.SID
WHERE (((MSysAccounts.Name)=[UserName]) AND ((MSysAccounts.FGroup)=0) AND ((MSysAccounts_1.FGroup)<>0));
BUT FIRST...you need to link to MSysAccounts and MSysGroups, simply go to the Tools|Options tabl and Check the 'Show' box System Objects. Then Link these tables from your .mdw file, usually it's C:\Windows\System\System.mdw.
--Jim