You can play co-op with friends through the Torii gate or after summoning them from a shrine.
The easiest way to play co-op with friends is to use the Expeditions option from the Torii gate, accessible from the starting point on your map. Expeditions basically start a co-op session going on a mission, and they don't need Ochoko Cups (more on those later). You can always create matches in a random group if all you're looking for is a walk-in co-op, but to play with friends you'll want to set up a custom match. From there, you can search for an existing part or create a new one.
The best way to play with friends is to set the summon condition to a secret word, which you can then share with the two friends you want to play with (there is a three player limit for groups in Nioh 2 , yourself included). You can set the condition to Friends, but it doesn't always work well. A secret word works every time, but if you use one, you may end up in someone else's room or send your friends to a stranger's house.
What if you're already on a mission and need the help of a friend or stranger? So easy! If you're on a mission, visit a shrine and select Summon Visitor. From there you can set conditions similar to an expedition (like a secret word), and even lock the second visitor location if you only want one person to help you (perfect when looking for help for strangers. ). The only caveat about this is that you will need to use one of the aforementioned Ochoko Cups to complete the summon, and it will cost one per person invited.
These are not terribly difficult to acquire, however. The Kodama Bazaar in the Shrine offers them, and the currency needed to purchase them is easily earned by donating equipment found in the field. These cups can also be found on defeated hostile ghosts (the Red Tombs of fallen players), so if you're confident in your combat prowess, you can farm them this way. Mugs are also used to summon friendly AI companions in Blue Tombs, but these computer-controlled allies tend to get killed quickly, so save your mugs for real people.
If you are looking to join a friend who uses Summon Visitor, you will need to be off a mission. Head to your Torii Gate and select Random Encounters. From here you can either help random players looking for help or join your friend's game (again, Secret Word is amazing here).
Remember: in Nioh 2, you don't quit a co-op session if you die once. Instead, you have a blue meter that runs out with every death. It is once this counter empties that the mission fails (the boss fights sideways, where the last person standing can try to overcome the encounter). Dark areas will replenish that meter a bit when cleared, and quickly resurrecting the dead will keep it. Don't be resigned if someone can join you and the bar is not going to run out completely.
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