Firstly, make sure that the user you are trying to add to your Publisher channel/ team has a registered user account. If they have not registered they will need to.
Go to your settings section and choose the channel you want to add them to on the left. Take the same email address they registered with and input it into the 'Add Member' section.
Add the email address and hit the Enter button. You now have a new user assigned to your Publisher Channel. (You have added them as an Editor)