The FurdLog points out that singing in a group raises oxytocin levels. He links to an LA Times article.
Researchers at the University of Stockholm found this effect. See:
Grape, C., Sandgren, M., Hansson, L.O., Ericson, M., and Theorell, T., Does singing promote well-being?: An empirical study of professional and amateur singers during a singing lesson. (Integrative physiological and behavioral science 2003 Jan-Mar; 38(1):65-74)