Grrr. Why do you mistakenly think there isn't ACT research on groups?!! (excuse the tone ... you would not believe how often I get asked this!).
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. (in press). Preliminary data on an acceptance-based emotion regulation group intervention for deliberate self-harm among women with Borderline Personality Disorder. Behavior Therapy.
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. (in press). Evaluation of Acceptance and Commitment Therapy for drug refractory epilepsy: A randomized controlled trial in South Africa. Epilepsia.
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).