See the recent press release from the American Psychological Association regarding the effectiveness of group treatments.
Links to Sister group organizations:
The APA Society 49, Group http://www.apadivisions.org/division-49/index.aspx
The American Group Psychotherapy Association AGPA http://www.agpa.org/
The Association of Specialists in Group Work ASGW http://asgw.org/
Council of Specialists CoS, APA http://cospp.org/