Yes, we have done research on forgiveness as a group intervention and we do get good statistical results. The very first journal article ever written on a forgiveness intervention was in a group setting with elderly women who had been hurt in family situations (Hebl & Enright, 1993). They became emotionally healthier as a result of this group effort. Here is the reference to that work:
Hebl, J. H., & Enright, R. D. (1993). Forgiveness as a psychotherapeutic goal with elderly females. Psychotherapy, 30, 658-667.
Other group efforts, as examples but not an exhaustive list, have included parentally love-deprived college students, people in residential drug rehabilitation, and men who have cardiac compromise:
Al-Mabuk, R., Enright, R. D., & Cardis, P. (1995). Forgiveness education with parentally love-deprived college students. Journal of Moral Education, 24, 427-444.
Lin, W.F., Mack, D., Enright, R.D., Krahn, D., & Baskin, T. (2004). Effects of forgiveness therapy on anger, mood, and vulnerability to substance use among inpatient substance-dependent clients. Journal of Consulting and Clinical Psychology, 72(6), 1114-1121.
Waltman, M.A., Russell, D.C., Coyle, C.T., Enright, R.D., Holter, A.C., & Swoboda, C. (2009). The effects of a forgiveness intervention on patients with coronary artery disease. Psychology and Health, 24, 11-27.
We do tend to find that individual interventions (one intervener and one participant) produce stronger statistical results than group interventions on forgiveness.