You can summon another player to help you at any Shrine. Select the "Summon Visitor" option and offer an Ochoko. You will not lose an Ochoko if no player is summoned, but you cannot summon another player without one. After doing this continue playing as summoning will happen automatically. It can take quite a long time currently to summon someone.
Matchmaking in Nioh is how the player is summoned to assist another. From the general overview screen select a Mission and hit the Matchmaking button to help another player in their world in that Mission. You can modify several different parameters for matching to find the ideal partner for your goals.
Online co-op is available via "Torii Gates" and Ochoko cups, making it so single player campaigners may summon in visitors from another world at a shrine. Similar to souls, the session ends if the host dies, but if the summoned ally is defeated another one may be brought in. Gestures and cosmetics are available to make this experience more unique, and players can trade equipment.
Another feature is designed specifically for co-op, with more challenging missions under the "Yokai Realm with a Companion" banner. In this mode, players can take on missions with more powerful enemies and Yokai as a duo, and share in a survival bar that depletes with each death and may be refilled slightly at shrines. Players in this mode can revive each other a limited number of times, making for entertaining moments.