How can I transfer a domain name from one SiteGround account to another?

In order to transfer a domain name from one SiteGround account to another, you need to post a support ticket with your request. You can do so via your Customer Area -> Support -> Open a Support Ticket -> Billing category

Note that the request must be posted from the customer area that currently has the domain name. In the ticket you should specify the domain name that you would like to be moved and the destination account where you would like the domain name to be moved to.