I wrote instructions for sharing a single VirtualBox virtual hard drive (vdi) among different mac accounts as well as moving the VirtualBox user setup/ preferences to a different account. You can see them at http://www.sfu.ca/~scitech/computing/nexgen_on_mac/
In a nutshell, (hopefully before you have created any VM's) Go into VirtualBox Preferences/General and change the default location from your user folder to the '/Users/Shared/VirtualBox VMs' folder. Create a new 'hard drive' in that folder. Then go to /Users/Shared/ folder and give read/write access to the "VirtualBox VMs" folder to users/groups that need access to it (Use 'Get Info', scroll to bottom, Under 'Sharing & Permissions' add users/groups and give them read/write access and then click on the gear wheel to Apply to Enclosed items). New users need to set their preferences to the same shared folder (or by transferring the Virtualbox preferences folder the new user will be setup).
Since a single windows virtual hard drive VDI takes 30-40GB or more hard drive space the savings can be important.
These instructions will work with Linux as well.
I was not able to find a guide to this possibly because of the many uses of the word shared 'shared' in VirtualBox (so apologies if this is in the instructions somewhere).