Before we get into the nitty-gritty, please remember:
You will need access to an internet connection and an active online membership (Xbox Live Gold or PS+).
Co-op mode will unlock after completing the intro sequence.
Co-op mode is not available cross-platform.
Once they accept the invitation, they will automatically join your game and appear near you with the character they customized during their own single-player campaign.
While in the co-op session, you will be able to interact with your friend's character to exchange items, like ammunition or medkits.
For an added challenge, you can also enable the Friendly fire option in the Gameplay settings.
If you run into connectivity issues while playing co-op with your friend, please follow the troubleshooting available in our articles dedicated to each platform: