If you want to play co-op in Remnant From The Ashes, you will first need to complete all of the tutorial portion of the game. After that point, you can create a co-op game by creating a game and setting it up for your friends only.
Once your game is created, you can then invite your friends to your game through the invite option on your system platform. The next way to play co-op is to simply ask your friend to join your game via the join option in the main menu as shown above.
If your friend uses the join game option, they will be able to search for both public games and games hosted by friends to join your game. Once your friend is invited or joined, they will be able to join your game and spawn immediately if you are on the game hub. But if you are on the move, you will need to rest near a crystal.
Related: Remnant: From the Ashes Armor Sets - How to Find Hidden/Secret Sets!
So if your friend is not spawning, that could be the reason, so try resting on a crystal and see what happens. After resting, your friend should appear in your game shortly after, and once they appear, you'll get a buff called teamwork.
This is a buff exclusive to co-op games that gives you and your friend bonus XP and increases damage resistance, making playing with friends a huge advantage.
For more on Remnant: From the Ashes, check out Remnant: From the Ashes Traits List Guide - How to Unlock & Where to Find! and Remnant: From the Ashes Weapons – How to Find Hidden/Secret Weapons and Melee Weapons!
