At the moment people just put links to them in the Modding sub-thread. We are expecting Steam Workshop support soon and then everyone can share maps. I would guess January/February time, so not too long now.
They have also said they are adding in-game support for map sharing too. So if the host has a map that the others don't have, then they can get it from within the lobby.
Really looking forward to map sharing as the current ones are getting a little stale. If you decide not to post them now and wait for the steam workshop support please don't forget to do it