Partying

As of currently, the only way to play with others is by giving your friend code or Steam username to other players then adding each other. The game will use the host's settings when creating a raid; settings changed while in a raid will not take effect until a new raid has been started.

If you are joining your friend and your friend isn't the host, you will not be able to join. This can be circumvented by joining while the host is in the raid.