There are two doors from which you leave Customs; next to one is an American connecting checkin counter, and next to the other is a United connecting checkin counter. If you want to park and go inside, you might arrange to meet at one of those.

See a map of the terminal. Customs time can vary greatly; in my experience O'Hare customs is generally quite slow in getting checked baggage.

AFAIK you have to take the train system to get to a parking lot.