depends on the game
some games have friend-invites, where someone will send you an invite to the game or you send it to them
some games have an id for your session, and you swap that with people who you want to join
and there's probably other ways.
Just to point out, once you are matched with someone in Switcher, you should reach out to them to say something like
"Hey, do you want to play now"
and then you need to talk to them about setting up a session
