The University of Michigan runs a physical activity promotion campaign every winter, called ActiveU. Faculty, staff, and graduate students can sign up, individually or in teams, and then self-report, through a web site, each physical activity episode.
We examined participation rates and found that those who participated as part of a team were 28% more likely to meet their self-set activity goals. That was after statistically controlling for factors like age, gender, BMI, and baseline activity levels. Details can be found in the journal article.