nathan: you don't, you'll have to share it out over a LAN etc
with the rasp the best way is to connect via LAN as of you can connect as much clients to the drive as you want and
the rasp has only 2 usb/1lan connection. i use a usb/lan bridge and it works not that bad. (2macs, 1rasp)
usb cant share connections so you really have only the way over a bridge (usb/lan or firewire/lan ...).
EDIT: if you do it over WIFI it will be pretty slow!