Would a guest network end up in the same room as well?
No, devices on the Guest Network would have access to the Internet, but they would not be able to "see" or communicate with any of the devices on your main or private network.
For example, if your printer was on the main or private network, any devices that were connected to the Guest Network could access the Internet, but they would not be able to access the printer or any other device on your main or private network.
In order for the Guest Network feature to work correctly on an AirPort router, you will need to have a simple modem.......the kind with only one Ethernet port. The Guest Network will not work correctly if the device that you might call your modem is really a modem/router or gateway device.