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. 

1. Click into the course in which you would like to add a user.
2. Select Settings on the course navigation menu. 
   
3. Make sure you are in the Course Details tab. Notice the Start and End dates located here (you may need to scroll down on this page). Even if there is no end date, the course automatically defaults to the term end date. 
   
4. Change the End date to a time in the future.
5. Check the box below that states "Users can only participate in the course between these dates."
   
6. Scroll to the bottom of the page and select Update Course Details. 
   
   

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.