Safety is the main issue here. If you come up on a group, stay in the back after acknowledging the rear rider. Hold your line and say "mind if I just hang here for a while?" Then at a stoplight inch up and say "hi, saw you guys. Where are you headed? Mind if I hang on a wheel for a while?" If you can prove you can ride smoothly you can begin to take a pull and people will trust you.

After a few miles - provided you can hold your line well, you will be trusted and welcomed.

Communication is the key. Chat at stops.