I've noticed the same thing. Some times we even encourage lone riders to ride with us. If we are stopped some place getting gas or just taking a break and we see a lone rider or two out and about we wave 'em down and see if they want to join us. The more the merrier.
lusone:No one has ever joined next to me, staggered, yes. Happens all the time and I do it as well. Better to ride in a group than alone, makes you more visible. I fall back if they do stupid crap though.