You can require clients to sign a waiver, contract, and/or terms of service when they create a new account.
Go to Tools > Profile > Settings
Scroll down to "Use your own terms"
Check the box and more options appear
Click "Add"
This works just like the editor in other places. More on how to use the editor here.
When a client goes to sign up or login (if they haven't signed the waiver) this is what they will see: