Typically you should plan for the BBQ assistant to arrive AFTER the arrival of food, and about 45 mins BEFORE you intend to start eating your first wave of BBQ dishes.
~10 - 15 mins setting up of fire
~5 - 10 mins for charcoal embers to be ready for cooking
~10-15 mins for first round of BBQ food cooking
~5 mins resting of freshly grilled meats and dishes
If you plan for guests to start eating at 7pm, then BBQ Service should start from 6pm - 9pm, so that the Assistant can do cooking for the entirety of your dinner.