Some suggestions:

1) Don't beat yourself up if others are going faster than you. It's just like running, we all put in our time and some payoff faster than others.

2) As others have said, you might want to compare your road tires to that of others after your first group ride. Notice the ones who are going faster but possibly same physical fitness level, they might have better slicks mounted.

3) Just ride! 30 miles might sound a lot now, but keep on doing your 10 miles and soon enough, you will improve your time/speed. It is like running, you must get back on the 'horse' to improve. Take the same elements of running and apply it here. When you went long distance, you just didn't do it on your 2nd or 3rd try, right? (fingers crossed) Same as in cycling, you build up endurance, power, and speed.

Good Luck!