Is there a cooperative mode in Helldivers 2?

Yes, the cooperative mode in Helldivers 2 is designed for a group of up to 4 people. If you are a newcomer, you need to complete the tutorial and the first training mission to unlock the cooperative mode. After about half an hour of learning the basics, you will be able to join your friends or invite them to your team to establish democracy together on the bug-infested planet.
https://youtu.be/zWPt8cqa_5I