20 to 30 is definitly too large for a training ride unless you all belong on a team and train together all the time. I declined an invitation about 20 years ago when they broke every promiss they made to me: no we don't run red lights nor stop signs, no we don't take over a lane on the road and stay within the bike lane, we always signal, we ride double pace line...

Don't be shy about voicing your concern. If you don't speak up, the group leader is not going to be aware of your situation. If you feel unsure about going down hill at 20mph, let your leader know so (s)he could adjust his/her speed. These group rides are meant to be FUN/safe/enjoyable and are meant to be a place to socialize with like minded people. A good group will accomodate their slowest and inexperienced riders.

And if their were 20 to 30 people, I'm sure their were others with similar riding experience as you. And you could form a slower/SAFE riding group. A seasoned rider should lead your group at your pace.

Shawn