We have had couples massages on a few occasions at resorts we visited. I have never found them any more erotic than a single massage, just very relaxing. I don't even remember thinking about my wife lying next to me. In fact the last time, her massuous was mala and I still didn't find it erotic. Maybe its just me.
I would think that group massaging with 3 couples would be ideal. You could have someone on each leg, each arm, and the torso all at once. It would be ideal if everyone knew the same techniques. Then the 2 people on the legs or arms could do the same thing at the same pace on the legs and arms. Are there 2 more couples out there that would like to try. The techniques we know are the ones taught in the book "The Massage Book" by George Downing.
Me and my wife have always enjoyed massages. But I have been specifically also been interested in also giving massages. It not sexual. Just the feeling of touch combined with providing real "non-sexual" pleasure has always been of interest. Touch therapy so to speak.
In general, we will be willing to join in for a massage session with interested couples.