To add someone onto a team, you must be within the team's talent 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 Talent Search page of the team, you can search and filter to find the right person for the position. Once you have settled on someone you think you'd like to add, click their team card:
Once you've focused on the person you'd like to add to your team, you will see his or her details on the right sidebar.
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."