Once you add an Editor to a site, they will be able to create and publish content in that site. They will not be able to access content in sites that they are not assigned to.
Go to the site you want to add an editor to
Click on Authors
Click Add Author
Add the author
If the author doesn't exist in ScreenSteps yet you can click the Invite new author button and you will be presented with the following screen. Just fill in their information and send them an invitation. Once they accept the invitation they will be added to the site.