You cannot add students to your D2L site, as they must be enrolled in your class. Once they are enrolled officially they will automatically be added to your D2L site the next time D2L syncs with UAccess (usually every 3 hours). However, sometimes you want to add additional instructors or TAs to your D2L site. Here is how:
Go to the Classlist from the D2L menu bar, click the blue "Add Participants" button, and select "Find Users".
Above "Add Existing Users", check the box to send an enrollment email to notify the person that they've been added.
Under "Add Existing Users", type the person's name or NetID into the search bar and hit Enter (or click the magnifying glass) to search. If your search comes up with no options, look for the person's UA email address in the UA directory
Check the box next to the person's name in the search results, select the role of Instructor, select the section number (any if there are multiple) and click the blue "Enroll selected users" button at the bottom.
You can also read the D2L Help pages on this subject here.