As groups discuss, even just one question, walk around the room and listen to the conversations. Believe that pairs or groups will do what you ask them to do. If they don't lunge into the activity, look critically at what you've asked of them. One of the hazards of group work is grades. Instead, throw out the grades and have groups present or publish their work for the class.
Grades often skew the focus of the activity and are very difficult to fairly assess.
On the other hand, when groups know that they will be sharing their conclusions or products with the class, they hold themselves accountable and participate. I form new groups at the beginning of every quarter. I also try to give students varying roles through the year. These changes develop individuals' social and academic skills as they work with others whom they might not have chosen. If copied, please give appropriate credit to author and web site.