enable steam overlay on the host device should fix it. Ran into this exact issue (with a different game), spent an hour plus looking for a solution.
there are 2 places to change the setting.
on the host computer
click on steam in the upper left.
Go to settings
go to "in game" on the left
enable "Enable steam overlay while in-game"
Then.
Right click the game in steam on whoever is hosting the game.
go to properties
under General you should see "Enable the steam overlay while in-game"
make sure that is checked too.
probably not a bad idea to do that on the machines of whoever is trying to join too.
Hope this helps