FileNet Workplace relies on Content Engine API regarding information about directory users/groups. Content Engine has server cache where information retrieved from directory server is stored. It might be that changes that you made in AD are not reflected there (yet). The easiest way to fix this is to restart Content Engine.
You can also adjust corresponding TTL entires in Content Engine configuration:
The workplace users/groups cache is managed by Cached Memory Timeout in process engine as mentioned here http://pic.dhe.ibm.com/infocenter/p8docs/v4r5m1/index.jsp?topic=%2Fcom.ibm.p8.doc%2Fpe_help%2Fadmin_hb%2Fwfa_user_cache.htm and the minimum time is one hour.