1. An apparatus, comprising: (a) a database within which is resident a dynamic group data structure, the dynamic group data structure including a group membership criterion and a set of member identifiers, each member identifier identifying a user from a plurality of users that meets the group membership criterion; and (b) a program, coupled to the database, the program configured to dynamically update the dynamic group data structure by updating the set of member identifiers to include those of the plurality of users that currently meet the group membership criterion as of the dynamic update.