Problem with LDAP users listed in multiple groups

I configured the connection to our LDAP server via webinterface(which works easy and perfect by the way).

The users are able to login and after I shared every group with “All users”(again via webinterface) the logged in users are shown in the roster. The problem is, most of our users are member of more than one group and accordingly get listet in multiple roster groups.

Is it possible to filter for every users primary group? The LDAP attribute gidNumber holds the primary group of every user in our configuration.