If you have an existing bridge on your server (e.g. br1) then it's really easy to use it in LXD. Be sure to choose an existing bridge when you do your LXD init. Then specify the bridge per container like so. The container will now contain an interface eth0 which is bridged to the host's br1 interface and can participate in your network as you would expect:
lxc config device add yourvm eth0 nic nictype=bridged parent=br1 name=eth0