The Friends of the ABC are an informal group of students who meet and discuss revolution in the back of a cafe. As such, roles are not entirely defined and certainly not designated by any set of standards. Rather, each member tends to define his own role simply by who he is and the position he seems to take in the group. In this way, you are correct in saying Enjolras takes on the position of the leader. He is considered the logic of the group. Combeferre represents the philosophy of the group. He balances Enjolras, and though much quieter and more subdued, he compliments him in leadership. Jean Provairre is considered the lover. Fueilly, an orphan who taught himself to read and write, takes on a role of generosity, hoping to deliver others in the way he delivered himself.
Courfeyrac is described as the center of the group, meaning he has a way of keeping everyone connected. He is the one who invites and most befriends Marius. Bahorel talks a lot, has a temper, a sense of humor, and is argumentative. Lesgle (or Bossuet) is unlucky but laughs easily. Joly is a quiet student of medicine, who is described as a bit of a hypochondriac. Grantaire, finally, is considered the skeptic and the cynic. However, he believes in and wholeheartedly loves Enjolras, and so he supports him. Subsequently, Grantaire is also considered the drunk of the group, and has his own song ("Drink with Me") in the musical.