At the moment when you wish to add domains, you just have to go through the sign-up form again, located on the MailShark website.
We then have the option to include the new domain(s) as part of your MailShark Domain Administrator account or as a separate account. We normally add all domains under the one MailShark Domain Administrator account so it's easy to manage all domains from the one account.
In future this process will be performed easily through the MailShark on-line ordering system, which we are diligently working on. Once ordered, the process will automatically create domains for you on the MailShark service which you can then access from the MailShark web interface.