If not in a batch, the OPU person takes out Drive Ups. Gather the stuff up in given the time and wait for the guest to arrive. If the OPU person is in a batch, best judgment is used whether he or she can grab the Drive Up. If not, they call on the walkie and someone else at the front will handle it. Between the service desk, cart attendant, GSTLs and SCO there's usually someone available. If not, grab a nearby salesfloor TM.
It's basically a team effort. It works fairly well because OPU picking for us is heavily concentrated at different times of day than drive up arrivals are concentrated.