LDAP Security Group members not listed

I have a Security Group CN that contain members I wish to authenticate against. I can establish the connection with LDAP server but when I try setting the Object Class to IsMember or memberOf and run “verify user mapping” I get nothing. I think I am close but am not quite getting it.
If I specify a CN with “Users” defined within and set Object Class to Users then I do get a listing so I can confirm that AD is responding.
Any suggestions?