Hello Jakob Øwre Vattengård,
Welcome to Microsoft Q&A forum.
Before we proceed, may I know what it says when you hover over the greyed out "Multiplayer" button?
If it says that multiplayer was disabled because of your account settings. Here are the steps for you to enable it.
- Go to xbox.com/privacy and log in with your account.
- Change to the "Windows 10 and Xbox Online Safety Settings"
- Set "You can join multiplayer games" and "You can join cross network play" to allow.
- Scroll down and select "Submit" to save the changes
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