Most randonneurs just ride at a pace that they find comfortable, and if that pace matches some company they can tolerate, so much the better. If it doesn't, or they can't, the group quickly dissolves.
Myself I enjoy the social side of the sport a great deal. I rarely ride at a pace that I find particularly challenging, preferring to simply find some company I enjoy and roll along together. Randonneurs are often very interesting people!
Also, I'm still learning how to deal with the longer distances and I feel more comfortable having a more experienced guy along to provide some guidance. The shorter rides, however, I feel more free to just hammer out whatever tempo my legs are capable of.