since it's truly a beginner ride, you're not going to want to follow too closely to other people since there's going to be a lot of room for error. i really doubt you will get dropped unless maybe you've only ridden your bike 1-5 miles at a time.

basically listen to everything your instructor / leader says, hold your line, pay attention and have fun.

i bet you a majority of the people riding along with you will be in general *work out* attire, nothing to worry about.