Yes, it is automatically supported: all you need to do is have 2 or more people on the same local network, and the signalling server will help them determine that they are both on the same LAN and connect directly, so game data doesn't even go over the Internet.
The Multiplayer object has a 'set bandwidth profile' action which lets you improve the gameplay quality when you have effectively unlimited bandwidth and zero ping, as is typical of most LANs. However don't turn it on automatically and don't use it for games that do run over the Internet, since it will probably make the game experience worse. It should probably be an in-game option, like a checkbox that the user ticks to indicate they are running a LAN game.
Players not on the LAN can still also join, so anyone over the internet can join your LAN game if the firewall permits. However don't use the LAN bandwidth profile in that mode either.