The place where we ususally meet is a parking lot that is located on the right IMMEDIATELY AFTER the Rickenbacker causeway in Key Biscayne. It is a pretty decent spot - there are some bathrooms there and there is also a little beach, so it is a pretty pleasant spot. It costs $1 toll to drive by car into Key Biscayne.
My favorite loop goes into Key biscayne and takes a detour into the residential areas once you are deep inside the key. On the way out, before hitting the causeway again, I usally go into Virginia Key (a park) and come back out to continue on the way to the causeway - just to add miles to the loop. Finally, after you go over the causeway and go past the toll booths, you could make a left into Bayshore Drive and ride all the way to Coconut Grove, where you just make a U-turn and head back into Key Biscayne, past the toll booths and the over the causeway. I remember that loop to be somewhere around 20 miles long, give or take a few miles. Since it is a loop, you can do just about any distance you desire. For example, if you decide to blow off going into Coconut Grove, the best place to turn around is right at the toll booths, where there is a small structure with a small parking lot on the median.
Traffic deep inside the Key is not bad, but it gets pretty busy on the way out, by the causeway and the toll booths. The most critical move is crossing the three lanes toward the median as you approach the toll booths on the way out. There is a good shoulder all over Key Biscayne, except for the road in the residential areas. Going into Coconut Grove, Bayshore Drive is a little more challenging, since it is a two-lane road with a fairly good amount of traffic. It is most definitely a single-paceline road. I guess the good thing about that entire area is that bike traffic is so heavy, that drivers (at least some of them) seem to be used to bikes.