It's my expereience when a group says the average 17 mph that means the average for flats with some stops. They don't generally count time climbing hills. And this is usually an off-the-cuff measure. The don't actually look at their average sped after every ride and figure this out. So, if you can do 17 mph on the flats, you'll probably be OK. And I agree, your average speed will probably go up even if you can't hang with the group all the time.