A Hat in Time's latest DLC, out this week, adds a new underground level to explore as well as online party play, which lets you muck about in groups of up to 50 players.
In the DLC, called Nyakuza Metro, you'll join the Nyakuza in search of riches, exploring nine chapters and a new Time Rift. You'll get to play with a new weapon, the baseball bat, and customise your gear via a new sticker system.
For online party play, you can play with your friends in groups of up to 50 players, or with online randoms in smaller groups of four, completing Time Rifts together. If you're playing with friends you can "hit, explode, shake and play catch with each other"—if you're playing with online strangers, they can't mess with you.
The DLC is free for anybody that backed the game on Kickstarter: if that's you, check the email you backed the project with.
For everybody else, it $7/£5.19, and you can buy it on Steam on the Humble Store. The GOG version is cheaper but doesn't have online party play, unfortunately, because it was developed using the Steam Networking API. Still, if you just want the new levels, the GOG edition is the way to go.
Correction: I initially said that the DLC added 10 new levels. That's not correct: it adds 10 new Time Pieces, which are rewards for completing chapters within the new Metro world.