How can we add additional questions to the registration form when people sign up so we can gather more information about their health and abilities?

You cannot add fields to the customer registration form at this stage but you can add question to the Welcome confirmation email sent to your clients.

Welcome Customer Email Template #

You can modify/customize this template in Settings > Email Templates

You have 2 options here

1. Add the questions directly on the email or
2. Create a Google Form(s) and include the link(s) on the email. You can see all the answers on a Google Spreadsheet.