After you've added a user to your course, that user will be able to login and access course information that is available to them.
If you have the "Email users when added to roster" course setting enabled, then users will receive an email once they are added to your course. The email will contain a sign up link.
If you've added a user to your course who doesn't yet have a codePost account, they must create one before accessing your course. New users can join codePost from https://codepost.io/signup/join.
The Roster pages have an option for you to send reminder emails to users that have yet to sign up.