Great problem to have. Anytime you have the chance to grow your business you should always grab the opportunity. First, you need to consult with a lawyer to help you write up a contract which you can use when hiring contractors or employees under you. This will ensure your protection as a business owner and avoid any legal and tax issues with your state. Although there is no 100% guarantee that you will be able to control everyone who will work under you, there are ways which you can protect yourself from potential issues. There has to be a non-compete agreement, at least for the area which you are operating.
Second, every new client needs to be contacted by you first, so you and them get to know each other and have a face to face relationship as well. All the payments for their sessions should be handled by you alone and not the trainer you will be contracting with. For every few sessions they buy you will need to train them as well, so you can keep the personal contact with them. Don’t let your trainers have the exclusivity. It’s a lot of work, but if you want to maintain control of your clients you will need to be involved a lot more than just an initial contact. Clients get attached to their trainers and become loyal to them and this is why you need to have constant contact with them.
Having appreciation days where all of your clients and trainers get together to have some fun will enforce the unity of your company and all of those who are involved in it (clients, trainers and sponsors if applicable). Also, since the area you operate is in a specific community it means that people know you and they trust you. This will make it more difficult for any trainer under you to go rogue and steel clients from you.
When doing interviews for hiring a trainer, make sure you vet them as much as possible and lay the ground rules to them. From that initial interview you should have a pretty good feeling if this person is someone you can trust and work with. Never assume anything and remember that trust usually goes both ways. Finding a good lawyer is half the battle, because he/she will play a big part ensuring the normal operation of your business, especially if you are going to hire other trainers/employees.
I’m sure others in this forum will give you some more ideas and tips of how to go about this. Good luck!