What do you mean by "share a host"? To share a folder from the host filesystem?
If so, do this:
Open VirtualBox, select the Virtual Machine, click SETTINGS
Select "Shared Folders", then "Add Shared Folder"
Choose path (in host) and mount name (in guest).
You can also check "Read Only" if you want.
In the guest, you can access that folder in Windows Explorer -> Network Places -> All Network -> VirtualBox shared folers -> \Vboxsvr".
Additionally, you could right-click the folder and select "Mount network drive" to assign it a letter (permantenly or not)
Last but not least, this feaure requires Guest aditions to be installed in the virtual machine