People can be enrolled in a class from either the Profile editor or from Training courses on the User tools panel.
From the Profile editor, select Training on the left navigation bar and follow the menus out to select a course you are offering.
From User tools, go to the Training courses tool, right-click on any course and select See the course's registrants. From there use either the Enroll a person link to search for a specific person or the Find people (advanced) link to search by criteria.
If you have set a maximum enrollment for your course and you hit that enrollment cap, you will get the message "This course is already filled to capacity, or will be overfilled with one added person. Add anyway?" If you choose to add someone, their status will be Waitlisted and they will be assigned a number for their position in the waitlist. (You can override the placement in the waitlist in case you need to move someone to the top of the list.)
If/when a person cancels, you will want to change the status of the person who cancelled, as well as anyone taking the spot from the waitlist. Right-click on the person, This training record and Edit record, and then change the status to Excused or Cancelled by volunteer, or perhaps Rebooked if they are taking a different class. Then change the status of the first person on the waitlist to Registered. Clicking the Pack waitlist link underon the left navigation bar will renumber the waitlist.
Only the registrant statuses Complete, Registered, Pending, Failed, Unconfirmed/tentative, Waitlisted and Incomplete count against a course's enrollment capacity as you have defined it in the course properties.