I really enjoyed riding in the groups that formed up--the people are why the ride is fun for me. I didn't stay tied to any one group, though, riding on or dropping off when our goals or speeds diverged.
I have used a space blanket, but mainly to lay on top of when stopping at night on the side of the road because the ground can be really damp. If you take it in a quart-size freezer bag you can fold it up and stuff it back into the bag without it later expanding and taking over your bike bag.
If the formal sleeping areas are full, find an out of the way corner in the cafeteria if it's cold outside. Under a table at 3 in the morning is a popular place!
Take some sort of alarm clock that you can use to set a wake-up call in case you get too sleepy to continue to your hotel or the control. I use a small kitchen timer so I just set the time I want to sleep--no trying to decipher AM or PM.
Be sure to include some serious walking or running in your fitness regime if you intend to do much tourist stuff in Paris before PBP. In 1999 I was very sore from the endless walking and stairclimbing from a week of "touristing" and going through the metro stations--starting PBP on sore legs wasn't the best idea I ever had.
Mark W