You can add a position to a shift on the schedule by clicking on the shift and then selecting "Edit Shift...". A box will appear that will allow you to adjust the number of positions left to fill in each job at this shift. 

Note: Unlike the box in the Recurring Shifts pane, this is the number of positions left to fill, not the total number of required shifts. If any of the positions are already filled at this shift, they will not be counted in the number in this box.