Make sure of the following:
- You are both friends on Steam
- Steam is online for both users
- The host of the game has checked the box “Allow friends to join”
You can also try restarting the game for both users.
You can also try restarting steam (make sure to exit completely).
If none of the aforementioned solutions resolve the issue you can try restarting your internet router:
- 1. Unplug your router or modem from its power outlet (don’t just turn it off)
- Wait 20-30 seconds, then plug it back in.
- Allow the device a minute or two to turn back on.
(More information about restarting routers can be found online if you need further assistance)