http://www.cisco.com/en/US/products/hw/wireless/ps4570/prod_technical_reference09186a00801444a1.html
You can either let everybody associate with the same SSID and then have the ACS override the default SSID-to-VLAN mapping on a per-user basis, so that user-to-VLAN mapping takes place with SSID being irrelevant,
or,
you can specify a list of allowed SSID's per user, and allow the default SSID-to-VLAN mapping to take effect normally.
If you want to go with the latter option, you'll need to set the device entries in ACS to Cisco IOS/PIX RADIUS (as opposed to RADIUS IETF which the docs indicate), and then create cisco-av-pair entries of the type "SSID = ssidname" in the user profile or usergroup profile.