This might sound awful, and offend some people, but those big charity (ms, diabetes) rides are NOT real group rides. They are filled with people who are starting out, just learning how to ride distances, and don't know anything about how to ride in a group. They are the worst groups to ride with. Most experienced cyclists don't ride charity rides. (ok, you guys can all kill me now, but I can take it.)

You should try to find a non-racing club, that has designated easy/medium/hard rides, and ride with the easy group. Introduce yourself to the ride leader at the beginning, tell him/her you are a new at group riding, and ask for help with learning basic group riding skills. No one is born knowing how to ride in a group, you have to learn. Admitting you don't know and asking for help is a better strategy than pretending or avoiding the situation.

Riding with groups is fun, you get a lot of benefit to your speed by drafting, you get to chat, you learn new skills, and you are forced to ride a little faster than you are comfortable with, and pretty soon you are going faster (and longer) than before.

Have a map with you, and be prepared to get home by yourself. Have proper clothing for the weather conditions, a little extra food in your pockets, 2 water bottles, and a well-maintained bike. Know how to fix your own flats and have the equipment to do so.

Here's a some stuff from the Diablo Cyclists, I used to ride with them before I moved to CO.
http://www.diablocyclists.org/Downloads/PaceLines.pdf
http://www.diablocyclists.org/DCtraining.html
http://www.diablocyclists.org/Traini...oupRiding.html
http://www.diablocyclists.org/Traini...GetFaster.html

Don't forget to have fun, and enjoy the sport!