With the Premium plan, you can invite up to 15 people to manage your signs, slides and content. To add additional users, you can simply invite them by entering their email through the MangoSigns dashboard. You can manage users through the profile icon in the bottom left hand corner of your dashboard under Team / Users.
From there you should see a Company Settings Tab, and a Users section in which you can click and see the option to Invite Users.
Once you click the Invite User button, you should see a window that allows you to enter the users email address and select the permissions you would like to give that user. See here to find out more about user permissions.
From here, the user should be sent an invitation email that allows them to complete the registration process and be linked to the account.