By default, both new and existing clients are allowed to self-schedule appointments. However, if you wish, you can limit online booking to existing clients only or people who know a secret passphrase.
Existing clients must enter the same email address and/or phone phone number you have on file in order to book an appointment.
Or, you can create a passphrase and only give it to people you want to be able to self-schedule online. You can change the passphrase anytime and email to select clients by clicking the 'Send Message' button on Clients > List.
Also, you can block individual clients from booking online by marking "No self-scheduling online" on their profile under Clients > Details.
And you can require clients to verify their email address before letting them book an appointment. We generally do not recommend this option.