It seems obvious to stuff a bunch of gamers into a single (digital) room and let them collaborate. But there are some lovely little touches to emphasize the painting, not the people: Each player is represented by a simple neon outline of a headset, preserving the sort of bodiless anonymity that draws the focus back to creating. You can even decorate each other's headsets, so expect the playful innocence of mustaches and fairy wings until less mature gamers inevitably invade Tilt Brush.
There are a few other features coming that aren't tied to multiplayer, like a poseable mannequin and letting players make crude animations, zoetrope-style: Adding doodles frame-by-frame on the outside of a wheel and "spinning" it.
Another tool on the horizon is the portal brush, which uses the HTC Vive's cameras to let you peer into the real world. Of course, these are all just experiments that may or may not make it into the game, but since the team keeps putting out niche features like the audio reactive brushes, we could see any of these come out in a future update.