You did not say what OS your virtual machines are running. But in any case, you can proceed just like you would for physical machines, ie. use network drives (SMB) for windows and NFS for Unix/Linux machines.
The thing to watch however is your network setup in each virtual machine. The default setup is NAT, which allows your VM to "dial out", i.e reach out to the internet, but does not allow anyone to "dial in" i.e. connect to your machine from the host or any guest on that host (or anyone else for that matter).
So for the above technique to work you will need to either use bridged networking for all your VMs, which essentially means that all connect directly to the physical ethernet adapters on your host (wired and/or wireless). To the outside world they look just like real machines and will get their IP addresses from your DHCP server. You can now start sharing and mounting drives NFS shares any way you like.
If you want to restrict the cross-mounting and sharing to just VMs on that one host, then you can do this: keep the NAT virtual adapter and add a second HOST-ONLY adapter. This means that your VM can now dial out to the internet and the other VMs on that host, but only the host (and other VMs on that host) can reach them. They are no longer reachable by other hosts. This is definitely better for security.
There is also a simpler way using just the default NAT setup. Just designate (or create) a folder on your host server that will be used to share files between your virtual machnes.
Then setup a shared folder on each of your virtual machines to point to that common folder on your host. That is done via the "Settings/Shared Folders" menu. You will want to make this shared folder permanent, and read/write since you want to share data between machines. Virtual box will make the shared folder available inside each VM. For windows it will be like a network drive.
To copy files between machines just copy them to the shared folder, and they will become available in the other VMs (and also on the host). In other words, you can now use that shared folder as a comon resource between your VMs.