Connecting two networks

We have a WAN (Windows NT 4.00)being manged by ourseleves. There is another network running Windows 2000, being managed by another group. These networks are physically not connected to each other but can be very easily either by connecting the switches or through a router.
I would like to know the proper procedures, both physically and policy wise, that need to be implemented, to allow access to the other network and vice-versa without compromising security. These networks will continue to be independently managed. Also keep in mind that other networks will need to be connected sometime in the future.

Thanks.