I have filed quite a few defects with Guest Server and AD.
One is : CSCsx66219 NGS: AD group mapping not working when querying domain controller
For this specific one, check the application log for :
admin Message: ldap_search(): Partial search results returned: Sizelimit exceeded; Type: Warning (2); Filename: adLDAP.php; Line: 503; Script: UserGroups.php
Either contact TAC for a patch or use a workaround which is to increase the MaxPageSize setting on the AD server
http://support.microsoft.com/?kbid=315071.
There are couple of other defects on NGS with AD, you may want to contact tac to process further or post here your logs so I can take a look.