Add fields in registration form / Appointments+

This is Stephane, from Paris, France.
I would like to know how I can add new fields in the registration form of Appointments+?
When clients register, so far they are asked for name, address, city, phone, emial and notes. I would like to add for example a field such as : "How many people are going to attend the session you are booking?".
Thank you for your help,