Much recent work has studied the behavioral and cognitive implications of dynamic inconsistency in the preferences of a single decision maker. Yet people are typically immersed in social relations, and often seek relief from their self–control problems by intensifying some of these interactions. Such is the case with self–help groups like Alcoholic Anonymous, Narcotics Anonymous and similar single–focus organizations in which agents are said to achieve better personal outcomes by simply sharing their experiences. Conversely, there are cases where group interactions can severely aggravate individual tendencies towards immediate gratification, as in some peer interactions among schoolmates or youths living in the same neighborhood. In this paper we therefore study how observing the actions of others affects individuals' ability to exercise self–control (resist impulses towards short–run gratification). The key difference with the single–agent case is that the informativeness of others' actions is endogenous, and depends on everyone's equilibrium strategies. The model, in which all externalities are purely informational, can give rise to multiple equilibria where agents' levels of self–restraint or self–indulgence are mutually reinforcing. More generally, we identify conditions on agents' initial self–confidence, confidence in others, and correlation between types (difficulty of the self–control problem) that uniquely lead to either a "good news" equilibrium where group membership improves self–discipline, a "bad news equilibrium" where it damages it, or to both. We also perform a welfare analysis to determine when –and for whom– group membership is preferable to, or worse than, staying alone. The model thus provides a formal theory of endogenous, psychologically–based peer effects or role models, as well as of the importance of group morale.