In the latest issue of Weekly Famitsu, translated by the folks at Gematsu, we got the first specific information about how multiplayer will function in Bandai Namco's upcoming Code Vein. Basically, if you're struggling, you'll be able to summon in someone to help you. It's currently unclear if you'll be able to summon in a specific friend from your friends list, or if you'll be matched up with a random player somewhere in the world.
Multiplayer – Clear dungeons with other players alongside your NPC partner. If you send out a distress signal, a player who has met certain conditions can come to your rescue. The rescue player appears alone, but can travel with you as a party character alongside your NPC partner. Your NPC partner’s Gifts will also have an affect on the rescue player.
Code Vein is due out at some point in 2018. Hopefully we'll have some more solid answers soon about who you can summon in to fight by your side.