If it is past the term end date, the +People tab will be grayed out. You are still able to manually enroll users with a few course modifications. By completing this process, you will be reopening the course. If you would like to close your course again after adding the user, simply undo the steps you completed to open the course.
You now will be able to add users to the course as normal. To re-close the course simply go back to Settings, delete the End date, and un-check the "Users can only participate in the course between these dates" option. Then, select Update course details.