There are two ways someone ends up in your Clients list. They book through your link, or you add them by hand.
When they book
Every client who books through your link is added automatically. Their name, phone number, and email come from the booking form. You don't need to do anything.
Add a client yourself
For walk-ins, returning regulars, or anyone you want to track without making them book online — open Clients and hit Add client.
You'll need their name and phone number at minimum. Email is optional. The notes field is for anything you want to remember about them — preferences, allergies, kid's name, whatever.