I price it so that each additional person is $20 an hour, with a maximum of 3 people. I spend the same amount of time planning for a partner / trio class as I would spend for one on one, and they know up front that there is a trade-off between price and customization.
I don’t do semi-private training as often as small group training. The difference, in my gym, is that semi-private training is a guaranteed session size with a known workout partner. Small group training is a guaranteed maximum class size, with less customization, and with varying participants. When I do semi-private training, the pair has always been friends who are of similar fitness level, and results have been good; some people do very well in a group setting because they push and motivate each other.