Switch to the list of the assignments to the team by clicking on the Assignments menu item at the top of the screen. Set the check box to the left of the assignment(s) to be assigned to an interviewer. (As a supervisor you can select multiple assignments to be assigned in a single action). Then, click on Assign and select an interviewer from the pop-up menu.
This will assign the selected assignments to the selected interviewer. The interviewer will see these new assignments on the dashboard of their tablet upon the next synchronization.
If necessary, you can use the filters at the left to narrow down the list of assignments to ones from a particular survey.