Microsoft Knowledge Base Article 840001 contains the following summary:
You can use two methods to restore deleted user accounts, computer accounts, and security groups. These objects are known collectively as security principals. In both methods, you authoritatively restore the deleted objects, and then you restore group membership information for the deleted security principals. When you restore a deleted object, you must restore the former values of the member and memberOf attributes in the affected security principal. The two methods are:
|•||Method 1: Restore the deleted user accounts, and then add the restored users back to their groups|
|•||Method 2: Authoritatively restore the deleted user accounts and the deleted users' security groups two times|
If you do not have a valid backup of the system state, and the domain where the deletion occurred contains Windows Server 2003-based domain controllers, you can manually or programmatically recover the deleted objects. You can also use the Repadmin utility to determine when and where a user was deleted.
Most large-scale deletions are accidental. Microsoft recommends that you take several steps to prevent others from deleting objects in bulk.