Chobby, the ingame lobby.
Chobby is on rapid, under the "chobby" tag. The development version can be obtained directly from here.
To download with pr-downloader use this command line:
pr-downloader chobby:test "Blank v1"
You can launch Chobby from spring start menu like any other game.
It is suggested to select "Player Only" script and it's a good idea to use Blank v1 as your map. Other small maps can also work pretty well.