Limit access of a SSID to a certain LDAP group

Hello, Maybe I'm just too stupid to figure this one out, but I have been googling around for several days trying to find a solution... I am running freeradius on Mac OS X Server. I have a Cisco WLC runnning several APs with multiple SSIDs. Everything is working fine, except: I have not found a way to limit access of a certain SSID to a certain LDAP group. I need to have different WLANs for different Users who are in LDAP groups. The user of group A should be able to use WLAN A but not WLAN B and so on. How on earth do I configure this? Does anybody have any experience with this? Thanks Qurt