Valid for Moodle 2.8. Updated on 7/15/2015. To add a TA you need to use the online form that's available from the Need Help? block that's (usually) in the upper right.
At the bottom click on the link to the online form for requesting an assistant. The resulting form is fairly simple to fill out. At point point you are asked for the privileges requested. One option allows the TA to add content to the course and act as a course builder. The other option merely allows them to views the resources and activities without being able to create new content.
You'll need to fill out a separate form for each TA you want to add. It will normally take between 12 and 48 hours to process the form and have the course participant list updated.