10. A system of matching users to user communities, the system comprising: memory for storing event data representing user-generated events that reflect user affinities for particular items represented in the electronic catalog and for storing information about a plurality of user communities, each of which corresponds to a respective subset of the plurality of users and comprises multiple users; and at least one computing device including a processor in communication with the memory, the at least one computing device operative to: identify a first plurality of items that characterize item preferences of a first user community of the plurality of user communities; calculate a degree to which the first plurality of items are related to a set of items in the electronic catalog identified by the first user; and based at least in part on the degree, determine whether to suggest the first user community to the first user.
10. A system of matching users to user communities, the system comprising: memory for storing event data representing user-generated events that reflect user affinities for particular items represented in the electronic catalog and for storing information about a plurality of user communities, each of which corresponds to a respective subset of the plurality of users and comprises multiple users; and at least one computing device including a processor in communication with the memory, the at least one computing device operative to: identify a first plurality of items that characterize item preferences of a first user community of the plurality of user communities; calculate a degree to which the first plurality of items are related to a set of items in the electronic catalog identified by the first user; and based at least in part on the degree, determine whether to suggest the first user community to the first user. 18. The system of claim 10 , wherein the first user community comprises users having email addresses associated with a particular organization.