Here are just a few of the studies on ACT done in groups (some of these also have individual sessions, but all have groups as a substantial part of the intervention):
Bond, F. W. & Bunce, D. (2000). Mediators of change in emotion-focused and problem-focused worksite stress management interventions. Journal of Occupational Health Psychology, 5, 156-163.
Zettle, R. D., & Raines, J. C. (1989). Group cognitive and contextual therapies in treatment of depression. Journal of Clinical Psychology, 45,438-445.
Hayes, S. C., Bissett, R., Roget, N., Padilla, M., Kohlenberg, B. S., Fisher, G., Masuda, A., Pistorello, J., Rye, A. K., Berry, K. & Niccolls, R. (2004). The impact of acceptance and commitment training and multicultural training on the stigmatizing attitudes and professional burnout of substance abuse counselors. Behavior Therapy, 35, 821-835.
Hayes, S. C., Wilson, K. G., Gifford, E. V., Bissett, R., Piasecki, M., Batten, S. V., Byrd, M., & Gregg, J. (2004). A randomized controlled trial of twelve-step facilitation and acceptance and commitment therapy with polysubstance abusing methadone maintained opiate addicts. Behavior Therapy, 35, 667-688.
Gifford, E. V., Kohlenberg, B. S., Hayes, S. C., Antonuccio, D. O., Piasecki, M. M.., Rasmussen-Hall, M. L., & Palm, K. M. (2004). Acceptance theory-based treatment for smoking cessation: An initial trial of Acceptance and Commitment Therapy. Behavior Therapy, 35, 689-705.
McCracken, L. M, Vowles, K. E., & Eccleston, C. (2005). Acceptance-based treatment for persons with complex, long-standing chronic pain: A preliminary analysis of treatment outcome in comparison to a waiting phase. Behaviour Research and Therapy, 43, 1335-1346.
Gratz, K. L. & Gunderson, J. G. (2006). Preliminary data on an acceptance-based emotion regulation group intervention for deliberate self-harm among women with Borderline Personality Disorder. Behavior Therapy, 37, 25-35.
Blackledge, J. T. & Hayes, S. C. (2006). Using Acceptance and Commitment Training in the support of parents of children diagnosed with autism. Child & Family Behavior Therapy, 28 (1), 1-18.
Lundgren, A. T., Dahl, J., Melin, L. & Kees, B. (2006). Evaluation of Acceptance and Commitment Therapy for drug refractory epilepsy: A randomized controlled trial in South Africa. Epilepsia, 47, 2173-2179.
Gregg, J. A., Callaghan, G. M., Hayes, S. C., & Glenn-Lawson, J. L. (2007). Improving diabetes self-management through acceptance, mindfulness, and values: A randomized controlled trial. Journal of Consulting and Clinical Psychology, 75(2), 336-343.
Luoma, J. B., Hayes, S. C., Roget, N., Fisher, G., Padilla, M., Bissett, R., Kohlenberg, B. K. , Holt, C., & & Twohig, M. P. (2008). Augmenting continuing education with psychologically-focused group consultation: Effects on adoption of Group Drug Counseling. Psychotherapy Theory, Research, Practice, Training, 44, 463-469.
Varra, A. A., Hayes, S. C., Roget, N., & Fisher, G. (2008). A randomized control trial examining the effect of Acceptance and Commitment Training on clinician willingness to use evidence-based pharmacotherapy. Journal of Consulting and Clinical Psychology, 76, 449-458.
Lillis, J., Hayes, S. C., Bunting, K., Masuda, A. (2009). Teaching acceptance and mindfulness to improve the lives of the obese: A preliminary test of a theoretical model. Annals of Behavioral Medicine, 37, 58-69.
Tapper, K., Shaw, C., Ilsley, J., Hill, A. J., Bond, F. W., & Moore, L. (2009). Exploratory randomised controlled trial of a mindfulness-based weight loss intervention for women. Appetite, 52, 396–404.
Flaxman, P. E. & Bond, F. W. (2010). A randomised worksite comparison of acceptance and commitment therapy and stress inoculation training. Behaviour Research and Therapy 43, 816-820.
Flaxman, P. E., & Bond, F. W. (2010). Worksite stress management training: Moderated effects and clinical significance. Journal of Occupational Health Psychology, 15, 347-358.
Fledderus, M., Bohlmeijer, E. T., Smit, F., & Westerhof, G. J. (2010). Mental health promotion as a new goal in public mental health care: A randomized controlled trial of an intervention enhancing psychological flexibility. American Journal of Public Health, 10, 2372-2378.
Bohlmeijer, E. T., Fledderus, M., Rokx, T. A., & Pieterse, M. E. (2011). Efficacy of an early intervention based on acceptance and commitment therapy for adults with depressive symptomatology: Evaluation in a randomized controlled trial. Behaviour Research and Therapy, 49, 62-67.
Brinkborg, H., Michanek, J., Hesser, H., & Berglund, G. (2011). Acceptance and commitment therapy for the treatment of stress among social workers: A randomized controlled trial. Behaviour Research and Therapy, 49, 389-398.
Pearson, A. N., Follette, V. M. & Hayes, S. C. (in press). A pilot study of Acceptance and Commitment Therapy (ACT) as a workshop intervention for body dissatisfaction and disordered eating attitudes. Cognitive and Behavioral Practice.
Luoma, J. B., Kohlenberg, B. S., Hayes, S. C. & Fletcher, L. (in press). Slow and steady wins the race: A randomized clinical trial of Acceptance and Commitment Therapy targeting shame in substance use disorders. Journal of Consulting and Clinical Psychology.
Morton, J., Snowdon, S., Gopold, M. & Guymer, E. (in press). Acceptance and Commitment Therapy group treatment for symptoms of Borderline Personality Disorder: A public sector pilot study. Cognitive and Behavioral Practice.
Folke, F., Parling, T., & Melin, L. (in press). Acceptance and Commitment Therapy for depression: A preliminary randomized clinical trial for unemployed on long-term sick leave. Cognitive and Behavioral Practice.
Biglan, A., Layton, G. L., Backen Jones, L., Hankins, M. & Rusby, J. C. (in press). The value of workshops on psychological flexibility for early childhood special education staff. Topics in Early Childhood Special Education.
Most of these studies are in the publications area of the site (and if one is missing prompt the author to get it up there).