Harvard abounds in clubs, teams, publications, orchestras, a capella groups, ethnic associations and sundry political associations. These activities can mean positions of prestige, ties of friendship, but they also mean commitment. Time and commitment. Behind every race, performance and magazine lies a large volume of meetings, rehearsals, practices, and planning sessions. Except for one loosely organized group--the Lowell Bell Tower Ringers. For them, every rehearsal is a performance, every performance a new composition. The only requirement for joining the group is a willingness to climb the eight flights of stairs and improvise on the ancient Russian bells. Any student is welcome to come watch and play, every Sunday morning at 1 p.m.