If you create an account for your clients with the email they provided you, they should NOT log in with Facebook. This WILL create a second account for them, and may cause problems for you.
The easiest way to have your clients register, would be the following:
- Send your clients to your subdomain http://yoursubdomain.fitclients.com (make sure there is no "www." in the URL)
- Click on "Create an Account"
- Click on "I'd rather sign in with email"
- Follow the steps to create an account