To add someone onto a team, you must be within the team's search page. To get here, either click "Search" in the upper-right corner of the Roster panel, or by clicking on an empty team position:
Once you're on the People Search page of the team, you can search and filter to find the right person for the position. To view see more information on each person, click on their "card":
To add that person to the team, you must first select the team position that you want to add them to. Select the drop-down, highlight the position, and click "PICK."