Hey, we've seen people not being able to join Co-op games. The team is currently looking into it as we speak.
To help us identify the reason / problem, we'd appreciate if you could describe the exact issues you're having and post them in this thread. This'll be a great help for the team fixing the issue. Thank you!
The primary solution currently is:
- Start Steam
- Go to Steam > Settings > Downloads tab
- Under Download region, select the same region as the other player you are playing Multiplayer with.
Thanks to community member @Mu-Draal for finding this! Legend behaviour 
Some current solutions suggested by the team are:
- Ensure you use: ctrl+shift+v when pasting the code as discord raw copy/paste has different formatting than AoD
- Make sure there are no spaces, tabs, or other non numbered characters
- If the individual you are connecting to has poor bandwidth or there's a great latency distance, then it will look like you're in the Main Menu, but in the background you're still trying to connect.