A search tool provided according to an aspect of the present invention maintains a cache data indicating which users are members of which dynamic groups. When a membership request is received requesting a set of dynamic groups to which a user belongs, the search tool examines the cache data to determine the set of dynamic groups to which the user belongs and sends the determined groups as a response to the membership request. According to another aspect of the present invention, the search tool may store an include list and an exclude list, respectively indicating the users to be included and excluded from each dynamic group. The lists are inspected in forming the set of dynamic groups to which the user belongs.