The white circles ARE the citizens. You should have as many white circles as you have population points IF you do not have any specialists (lower left of the screen) assigned. By clicking on a square that does not have a white circle you will place a citizen on that circle. If you click on a square with a white circle that circle will go away and a specialist will be assigned. Thus, in order to move a worker, click on the circle you want to move THEN click on the square you want to work.