I would hope the first group is well above the C limit. The ride description state that the last 20 minutes is a race so it will be a full out effort for all riders that feel like racing.
From the ride description:
Are you ready to join one of the largest and fastest-growing group rides on Zwift? The Americas Hump Day Ride (AHDR) is two events in one, starting with a moderate effort of 2.3 - 2.8 w/kg. Use this time to warm-up, chat with fellow Zwifters, and maybe decide on a wheel or two you’re ready to get on.
Because the AHDR gets spicy after the leader announces the start of the After Party: a 20-minute race. Be sure to listen to the ride leader’s instructions, before digging deep. Or not. If you want to keep it casual, it’s all good.
During the After Party the ride leader acts as a sweep, riding with the After Party cruisers, so everyone finishes together.