I've been doing some research and I'm finding that there is no support for a "memberof" type attribute for user objects.
Use slapo-memberof which is available in OpenLDAP 2.4.x:
http://www.openldap.org/software/man.cgi?query=slapo-memberof&sektion=5&apropos=0&manpath=OpenLDAP+2.4-Release
When I query groups that have "memberuid" attributes, I get the group name but not the members.
Ciao, Michael.