I did a little bit of research over the weekend, which I thoroughly enjoyed BTW, based on this topic. I searched for and watched a number of "group sex" porn videos, especially the bisexual variants where there really are no restrictions on who puts what where. It's funny but i noticed that most of these people in the videos aren't really focussing on the sexual act that they are in the middle of. They all seem to be looking around at the others to see who's looking at them.
Maybe it a product of the fact it is a staged porn event, but were I to engage in group sex, I would still be very much focussed on pleasuring the person I'm with.