Being smooth is the key....at any speed. If the group does not know you are new to group riding, let them know.

Learn to maintain a steady gap to the rider in front of you without using brakes. Use body position (sit up to slow down a touch), bike position (slide out of line a few inches to slow down even more), and soft pedaling (smoother accelerations) to stay smooth. If the gap opens, slowly increase speed to close it (don't panic and hammer)

Stay alert and focused, so if there is an issue in front of you, you can react. Even if you need to brake hard, brake only as hard as necessary (to help the guy behind you).

Finally, be prepared with tools to fix a flat. Even if you aren't skilled at fixing flats, having the tools shows you are trying.