Homegroup is only a Windows 7 thing, a Vista machine can't connect to a Homegroup.
There are a couple of ways you can share the USB cellular internet connection. First, you can buy a router that will accept your usb cellular device and then you can connect both machines to the router wirelessly. The Cradlepoint CTR35 Portable Router
is such a router but you would need to check to make sure your usb device is compatible with it.
A cheaper way to share the internet connection between those 2 computers would be to install the Connectify
software on the Win 7 machine which will let your Win 7 machine act as a wifi hotspot and allow sharing of it's internet connection with other computers via wifi.
Once you have a network established by either method above, you should then be able to share your external HDD over the network.