To send a Wave, swipe right on the Hallway or tap the nine dots square at the bottom left of the screen. Then tap the 👋 button next to the person you’d like to chat with.
They'll get a notification that you said hello, and know that you're open to chatting. If they are too, they can join a private room with you — open just to the people you waved at!
You can also Wave to someone directly from their profile! Simply go to their profile and tap the "Wave" button at the bottom.