There’s no INVITE button at the bottom of the Multiplayer screen. The only reliable way to join a game right now, it seems, is to make your game public, invite your friend from the Steam Friends list, then close your lobby again.
Actually even with “Invite Only”, I was able to invite my friends by hitting “R” on PC. This brought up the Steam friends list like before and I could invite them. I seem to recall this used to be “T”.
I can confirm, its always been R on PC. I dont believe T has a keybind in game unless you rebind something to it.