Cisco Secure ACS has the following limits with respect to group mapping for users authenticated by a Windows user database:
Cisco Secure ACS can only support group mapping for users who belong to 500 or less Windows groups.
Cisco Secure ACS can only perform group mapping using the local and global groups a user belongs to in the domain that authenticated the user. Group membership in domains trusted by the authenticating domain cannot be used for Cisco Secure ACS group mapping. This restriction is not removed by adding a remote group to a group local to the domain providing authentication.
http://www.cisco.com/univercd/cc/td/doc/product/access/acs_soft/csacs4nt/acs33/user/qg.htm#wp940528