There are three ways to do this. All of them involve changing the network type for your guests.
You can set up a host-only network. Your guests will have a virtual LAN that they can use to communicate with each other. The host will also get an interface to the LAN, so the host and the guests can all talk to each other. Virtualbox will let you set up a DHCP server for the LAN.
Alternately, you can change each of the guests to use a bridged network. With bridging, the guest will have direct access to your host's network connection. So each of the guests will be just like a real computer on your local network.
Last, Virtualbox 4.3 added a NAT network feature. It's currently marked as experimental, and I found it to be a little flaky, but you can try it. NAT network also provides a virtual network that the guests can use to communicate with each other, like the host-only network. NAT network also provides a NAT connection to the host's physical network. You can set up port forwarding to let the host (or other computers) access the guests.