When logged out, if you press the Login button, you can see there is a link where you can register a new account on Multicraft (If you don't see that, submit a ticket to us).
After registering a new account, you can allow them to access any of your servers by going to the Advanced > Users menu on the left bar, then search for their email in the text box. From there, you can give them "Full Access" or "No Access" for FTP (Read only is the same as No Access). You can give them control panel access roles as well where "Administrator" is the same access level as your main account and is required for server restart capabilities (any other role cannot restart your server).