First, make sure you have followed the directions on the left
1. To include complimentary registration in your event pricing
2. To navigate to the Event Manager.
Once you are at the Event Manager page, find the event you want to register someone for and use the drop-down on the right to find "Special"

Use the green button at the right to add a registrant, and then search.

Once you have found the contact you want, select "Complimentary" from the drop-down list. (You must have already included a complimentary price in your event setup - see directions on the left).
If you are assigning a Special Price, just select the "Admin only" price instead. At this stage you could assign any price that exists in your event setup to this individual, so you can override the price that the system assigns.

IMPORTANT: MAKE SURE YOU CLICK "SAVE" AT THE BOTTOM OF THE PAGE, AND AFTERWARD CHECK THAT THE RIGHT CLASS WAS ASSIGNED
Now you can either register the individual yourself, or they can register themselves, using the usual registration process.