It just takes a few simple steps to assign a client to a sub-trainer that you have added to your account, starting from the "Clients" section of your Trainer Dashboard:

Step 1: Click the gear icon (under "Actions") next to the client of your choice. 

Step 2: Scroll down and select your preferred sub trainer's name under the "Client Trainer" header. 

Step 3: Then click "Save Client" to confirm your choice and assign the client to the sub trainer. 

That's it! Your client is now assigned to the sub trainer of your choice.