virtualbox.org

I have a LAN guest network of 6 Win servers, all can talk to each other. NIC #2 is "internal network" on the guests.
one of the 6 have NIC #1 configured for LAN and can access the internet.
I can't figure out how to configure the other 5 servers to access the internet either directly or (preferably) via the one with access.

See if this helps. Principle is the same as when using a Domain.
Setup Windows Domain in VirtualBox

As usual the best thing to ask yourself is "How would I do this with six physical machines on a hub/switch"? The answer to one usually points you to the answer for the other.