This kind of thing has happened to me plenty of times. It actually took a long time for me to hang with the group. Now, I pull with the best of them and don't get dropped. Here's what to do so that you can "stick" to the pack:
1. Learn proper drafting technique... this means not hesitating on the turns and plowing through them as fast as possible. It also means knowing how to anticipate surges that would otherwise drop you: if you start to see the people in front of you breaking away, don't let it happen. What can also help is knowing the course that they take in their rides, when you know it well enough, you can ride faster still. All that being said, it boils down to never getting out of a draft and getting pushed into the wind by other riders.
2. Get faster by working like a *******. Pretend you're the wolf in the three little pigs story, and if you're not huffing and puffing like you're gonna blow the brick house down, you should be. Do this often enough, and you'll increase your power output, strength, efficiency, and endurance. Lactic acid is a friendly burn, besides.
You can also train by yourself... so some interval work and have proper rest periods, and you'll get faster. Riding a ton helps, too.
Good luck