I have used a "temp" agency. Usually I'll bring the prospect to them and they'll process him, I-9, WC, witholding, etc, all of it. Costs vary, but they take care of all the headaches, I pay them and I'm legally seperated from liabilty unless there is gross negligence or breech of contract on my side. I'll use them during the height of the season when I need to beef up the crews. I won't need the "temps" during the off season.
I've managed to keep the cost to around $13.00 an hour per employee, I don't care what the agency pays the worker - the agency takes care of everything else and I stay legal. Even $15.00 per employee is worth not having to deal with all the book keeping and paperwork.