A system for determining recommendations which are likely to be relevant to a user's current tasks. The determination is made based on the user's grouping behavior as applied to collected references, documents and information objects. The recommendations are with respect to one or more of the “target” user's existing groupings or categories, and can be of information objects, categories of information objects, or other users who have created similar groupings of objects to the target user's groupings. The basis for recommendations is the discovery of “matching” categories for a “target” category, where a matching category is one that includes at least one object also included in the target category. Objects which are in matching categories but not in the target category are candidates for recommendation. In this way the recommendation system aids the target user by taking advantage of objects gathered and grouped by other users over time.