You will be fine. There should be lots of communication on the day, you'll all agree hand signals and as it's a steady pace there should be time / space to get a feel for it. There is a difference between group riding and say a fast paceline on the track (where there really aren't any room for errors). I had my first experience group riding on a HDIJ so it's all good. Just hold you line, communicate and don't make any sudden moves.