How to Join Friends and Fix Launch Game:
The simplest way is that your friend invite you. Once one the character selection screen, he can invite you by creating a private lobby. Then, use Steam interface (shift+tab, right click on the person you want to invite, and click on "invite a friend to play").
How to Fix Launch Game?
Make sure to close Steam. Follow this steps to disable the Compatibility Mode:
- Browse to your Steam installation (Usually C:\Program Files (x86)\Steam)
- Right click on Steam.exe (or bin\steamwebhelper.exe), choose Properties
- Click on the Compatibility tab
- Uncheck any boxes that are checked, and click Apply
- Then, click the "Show settings for all users" button
- Again, make sure none of these boxes are checked, and click Ok
- Click Ok to close any open dialog boxes.
- Double click on Steam.exe to launch Steam.
- Re-test the issue.
Once this mode is disabled, you can launch the game.