Yep, let them set the pace, but also know where you stand. You have your own boundaries as well.
Also, enjoy. One of the best sexual experiences of my life was myself and a MF couple. He and I didn't do anything with each other, but focusing on this amazingly beautiful woman who was lucky enough to have TWO guys making her the center of their focus AND that I was lucky enough to be one of those guys...
Great experience all around. One of my best, in fact. Focus on the moment and not the "situation", if that makes sense.
Connection is the path to passion. Passion is not the path to connection.