Everyone gets dropped sometimes.
On hilly terrain, groups will often regroup after long climbs to accommodate a variety of climbing abilities throughout the group. Start with a group with a no-drop policy to be sure someone sweeps the back and checks up on everyone so you don't get lost if you do find yourself trailing behind. Talk to people about all the different group rides in the area so that you know if your abilities and equipment are well suited for the ride. Then pick the group accordingly. Also, don't worry about your 20 mile speed if your group ride is going to be 40 miles. Well rounded training may include working your butt off for 20 miles to go as fast as you can, but it will also include some challenging distances at whatever pace you can maintain for that distance. Riding in groups can make you faster because there are natural surges in pace that happen in a group that you'll need to respond to. It's kind of like doing intervals. That may help you by improving your recovery time which can translate to overall speed increases. It may also force you to hit speeds in the chase that you didn't think you could do or that you may not be able to do on your own (the group gives you extra motivation).

Some flat land average speeds can be good guidelines for finding out if your fitness level fits loosely into that of the group, but when you aren't actually riding on the flats much at all, most of that goes out the window, so you'll never really know if you can hang with the group until you try.