It helps to have all of your rate and service offering written down for clients to choose from. To switch them over you’ll have to give them some kind of discount from what they are currently paying, but of course make sure that the new rate adds up to more than you were making off of one.
Here’s an example:
Private training $100
Semiprivate training with 2 clients: $75
Semiprivate with 3: $65
Or just $75 for semiprivate regardless of the number.
This also gives you the “liscence” to increase your private training rate if you are so inclined.
Now for selling the new offering: explain the them the benefit of having training partners. We all have them for our personal workouts and they help with motivation, friendly competition and increase attendance. I had a group of 3-4 clients that I trained for years who really pushed each other to radical heights: leg pressing. 1000+, pull up contests over 25 reps, chest pressing 100 lb Dumbbells and the like. They maintain relationships even today.
Partnered training truly does bring more value to the clients workouts as long as you have good partners.