Can you elaborate a little more on your question? If your question is about marketing small group training, you could always place an ad, make flyers, hold demonstrations, tell your existing clients, etc… Word of mouth is wonderful if you have a following.
If your question is about teaching small groups, it’s very rewarding. You are able to teach a class, yet focus on the individuals. There are numerous classes you could do (I teach TRX and bootcamp classes this way) to keep it interesting and challenging. In addition, it can attract more clients who want individualized training.