Everyone has wonderful advice.

They know they're slow, and they likely feel bad about it. I started riding with co-workers after work. It was clear that one co-worker (whose goal was to train) clearly wasn't getting a workout because we were too slow. I had an accident that prevented me from riding for a few months, and the rides just sortof went away -- but in a way it was a relief because the two of us that were slow felt bad and the fast person obviously didn't want to hurt our feelings. I'll admit -- when the fast person would ask if the pace was right, I often reacted huffily which was unfair because the fast rider was just being courteous but my reaction was more because I knew I was holding him back. We had these issues and we'd known each other nearly a decade. I can't imagine how much trickier it is with someone you're just meeting!