Go onto Institutions > Timetable > Timetable Administration > Timetable Slots.
On the Timetable slots page, you are able to take several actions, use the filter panel at the top of the page and filter your timetable slots by subject, course/class or year group. This will help you easily identify lessons that have the same teacher or are in the same room.
Tick the timetable slots that need to have more staff members added onto, then click on 'Change staff'.
A slide over will then load with a drop down that populates all staff members. The quickest way to add a staff member is by typing the staff members name and then clicking on them.