A Web based system provides informational services for assisting customers in selecting products or other types of items from an electronic catalog of a merchant. Users of the system can create and join user communities, such as communities based on user hobbies, localities, professions, and organizations. The system also supports implicit membership communities that are based on email addresses (e.g., all users having a "nasa.com" email address), shipping/billing addresses, and other known user information. Using purchase history data collected for online users, the system automatically identifies and generates lists of the most popular items (and/or items that are becoming popular) within particular communities, and makes such information available to users for viewing. For example, in the context of an online book store users of the nasa.com community may automatically be presented a Web page which lists the bestselling book titles among nasa.com users, or may be sent email notifications of purchase events or hotselling books within the community. Another feature involves automatically notifying users interested in particular products of other users (preferably other members of the same community) that have purchased the same or similar products. For example, in one embodiment, when a user accesses a book detail page, the detail page is customized to include the names and email addresses of other members of the user's community that recently purchased the same book.