My new strategy for group rides where I don't know most people is to ride back with the sweep. Once we get some time and km in, it is easy to get a feel for the skill level and speeds of those in the group. If the group is faster/willing to risk a 50 over, then I keep back with the sweep. If the group is full of newer and slower riders, then I decide if I want them in front of me, or if I want to pass on the straights and advance up in the group of riders.
Granted, the above doesn't work if your bike is completely out-classed by all the others. I can't count the number of times I got 'screwed' on my 250, when I'd hang back, and then end up getting stuck, as the guys in front would corner well below the limit, then gun it up to easily ticketable speeds as soon as the road got straight, only to park it in the next corner again and again.