Weight Loss Competition between clients Need IDEAS

I have 13 women, some clients doing a 7 week body challenge with them. Obviously I will be measuring progress by body fat%, measurements, weight and overall body transformation pictures.

Starts May 6th-June 24
Cost is $50 and they will win pot of money as well as 5 sessions with me and meal plan from certified natural chef and some other goodies.

Concern is I have a lot of women who are thin and just need to gain muscle. How do they compete against a woman who needs to lose 25 pounds? Should I put them in groups or partners? Any suggestions would be appreciated.