Can you not play with demo players from full version.

[Raven-Symonés Discography]

title says it all. trying to play with friends but i'm the only one with full version. cant find lobby and can't join or invite.

[Kona❤Kona]

Correct. I'm not sure if you can get the demo while owning the game.

[Clyceer]

There's a post in https://steamcommunity.com/app/2132850/discussions/0/4335357742819081463/ that describes how to get the demo again while having the full game:

Originally posted by 󠀡󠀡:

You can force Steam to download.
1. Press Windows + R
2. Type 'steam://open/console'. This will open a new tab in your steam called Console.
3 In that tab you type: 'app_install 2598750'

Seems to work for me, but definitely assumes a Windows computer.

Edit: Also, the Console tab will be gone again the next time you open Steam. It's not a permanent change to your Steam UI.