How do I set who works on service?

Open the Directory from the side nav, then Company directory, then Your people.
Press the Works on cell on a person and pick Projects, Service or Both. It saves straight away.
To set many at once, tick their rows. The line above the list then shows how many are ticked, Set and Clear. Press Set and pick Works on, or What they service to record the trades they work on the Service side, then press Save. You can set a pay grade the same way.
Filter has a Works on group and a What they service group, and Download carries both columns.

Worth knowing

What they service is what you recorded, not a licence check: the tickets they hold are still on their record. Only somebody with company administration can change either.