Adding new users to sites should be limited to essential access - managing users that are not using CampusID should be avoided if at all possible.
Existing users
Go to the dashboard for the subsite, select "Users" / "Add New"
Under "Add Existing User" start entering "E-mail or Username" - the user should appear in a pop-up - select the user from that list. Select the appropriate role - the lowest that will suffice for that user.
It is optional whether or not to send a welcome e-mail.
Click "Add Existing User" button.