Adding another User/Admin on Multicraft is helpful if you want to give another person access to your console/files. Do not do this if you do not trust the person, as they will have full access to your server.
2) Ask for the username that they used to make an account, for example Steve.
3) Go to the Users portion of your Multicraft Panel
4) Type in their username into the space and hit enter, change their FTP access to Full Access if you're wanting them to be able to edit your files, and change their Role to whatever you wish.