Group marketing uses the psychological mechanisms underlying group influence to drive customer behaviors that are beneficial to the firm. It is predicated on the firm’s ability to guide two necessary and sufficient conditions: (1) a customer’s awareness of an affiliation with the focal group and (2) exposure to group norms. By examining what it means to be affiliated with a group; determining how group norms are inferred, applied, and maintained; and testing a wide variety of ways in which these conditions become manifest, this research demonstrates the theoretical foundations of group marketing. Groups influence purchase behaviors by altering information and identity appraisals during decision making. Time in a purchase domain emerges as a critical determinant of the strength of group influence. Although previous research has suggested that social influence diminishes over time, a longitudinal field study and an experiment reveal that this prediction holds only when information appraisal dominates; an opposite effect arises when identity appraisal dominates. Group efficacy strengthens, but product price weakens, the effects of groups on purchase behaviors.