NOTE: Domain control is in limited private beta.
If you are attempting to create a new account with a domain that has domain control enabled, you will see this message
Or, if changing to a new email address from the Account Settings menu, you could see this:
This means that if you are a part of this organization, you will need to ask for access into their Calendly account. You can request to join their organization using the button in the message.
If you want to create a separate account, you can still do so. However, you will need to sign up with a personal domain instead.