Multiplayer! Well, yes, but actually no. Kinda. You can start and join multiplayer missions, but it should still be considered work-in-progress, a lot of things are still untested, and can very easily go wrong. Also, this is considered a “minimum viable product”, i.e. the bare-bone version. Functionality will continue to be added over the next updates. There will be a ‘proper’ multiplayer launch once all the kinks have been ironed out.
You can read more about multiplayer here: https://forum.chickeninvaders.com/t/multiplayer/18380
More details about this update can be found here: https://forum.chickeninvaders.com/t/early-access-version-91/18401