Where customers come from
There is no "Add customer" button. The customer list is filled automatically in two cases below. One customer = one email; further payments or bookings with the same email are attached to the same record.
Way 1: Payment (link or checkout)
When someone pays for your offer for the first time via payment link (or QR) or checkout session and provides email (or logs in - then email from account), the system creates a new customer in your org. Stored: email and - if provided - name, phone. Each further payment with the same email is assigned to that customer; in customer details you see full payment and subscription history.
Way 2: Schedule booking
When someone books a slot in your schedule (public link) and provides email (required for booking), the system creates a customer (if the email is new) or attaches the booking to an existing customer. Data: email, optionally first and last name. The Customers list therefore also shows people who only booked (e.g. pay-after-visit) even if they have not paid yet.
Summary
In the system a customer is tied to data from payment or booking. The org customer list is everyone with at least one payment or booking in your org. In the panel you see contact data and aggregated stats. Customers = everyone who gave email at payment (link, QR, checkout) or at schedule booking. From the panel you can browse, search, filter and open details (payments, subscriptions). On Premium and Enterprise you can also add a customer via API (e.g. lead before first payment).
Leads before first payment
With Collect company data at payment and Lead collection enabled on the offer, form data (e.g. company email) may create a customer record before a successful charge. An email marked as lead may prevent collecting the same address again on the payment page (emailAlreadyLead). On Premium and Enterprise you can also add customers via API.