Multiplayer functionality is handled differently by every title. Most content uses some sort of invitation or friend list system. Usually you simply need to invite another station via their Steam ID. Please note that it is a Steam Requirement that an account needs to have spent at least $5USD on it to be able to add other people as Friends.
It is also beneficial to ensure each stations Steam account is added to each others friend list. This will create simple invite lists for some content.
Right now there is no central way to join multiplayer titles from SpringboardVR, but we hope to offer this functionality in the future.
For information on multiplayer functions for specific content, email us at firstname.lastname@example.org.