Someone on your friends who has it randomly invited you. You used to need someone's email and steam account to invite them now you can just invite anyone in your friends with a clock of a button. I went and invited all my friends I thought might be interested without telling them.
It feels random, I don't have VR or play any similar games, none of my friends invited me or play it themselves, nor do they play those types of games either.
My guess is it was a shotgun invite to get momentum and play testing.