Don't slam on the brakes
This is the number one tip for riding in a group. If you've got riders behind you, don't grab the brakes. Instead, slow down gradually, while communicating to the people around you. You can call out to say you're slowing down, or hold a hand out behind you. Better yet, for a belt and braces approach, do both.
Keep attentive
The best thing about riding in a group is the energy you save riding behind other riders. Protected from the wind, you can save about 30% of your energy. To get that benefit, though, you need to ride close to the other riders. Aim to ride between 20cm and a metre behind the rider in front of you. When you're that close to one another, you need to keep focused. Keep an eye on the road and on the riders around you, so you're ready to react if they brake or change direction.
Use warning signs
When you're riding in a group, you can't always see what's coming up ahead. So, if you're on the front, signal to the riders behind, both verbally, and with a hand gesture. Point out potholes, speed bumps, parked cars, and anything else that could cause you to change direction.