TUG in the coming weeks, as the devs have said that they're preparing to push updates out to get more people playing together. Survival and creative modes will both be getting multiplayer love, although the team has asked for players to help spot and report bugs during this testing cycle.
The team did warn that these builds will start small: "Multiplayer in this iteration is still fairly early, so don't expect crazy furry parties just yet. With more time and more tech, the experience and player count will improve. As to what that count is, we still cannot say."