At the present time, we are using it in a Camp Placement facilities that house approximately 60-110 minors per. We have close-ended groups. As I said, it is in its infancy. Since we have camp placement orders ranging from 3-6-9-12 months, closed-ended is the only way to go for us at the present time. As of July 2008, they tell us all camp orders will be for a minimum of 9-months, so closed ended will be our format.
Considering your issues and need to run open-ended groups, it is a blessing to be working with the same curriculum. Having the youth attend open-ended small groups shouldn't be that much of a problem, in that there is the understanding that there will always be the need to restructure to group expectations. This is doable. After all, the MATERIAL and the teaching to problem-solving skill development is the evidence-based practice, yes?
I would suggest you break down your "monster" into bite-sized chunks (AKA bodies). You can keep that small group together and add to it as bodies come and go. You may have some that make it all the way through three months of lessons.