There's lots of good advice here and I'd just like to add one thing. A lot of people are physically strong enough to hang on when the pace goes up but they give up mentally because it seems too hard.

Keep doing the group rides and when you feel like you are dying grit your teeth and hang on for as much longer as you can. You may find that the pace eases pretty quickly or that after a minute or two it doesn't seem that bad any more. Either way it'll make you stronger, physically and mentally, and if you keep going back and doing this eventually you probably won't get dropped.