On the following page of the Coral Island beginner's guide, we have provided information about the multiplayer mode.
This page of the guide was written based on the Early Access version of the game. It is up to date as of October 12, 2022.
The developers have plans for a multiplayer mode, but it was not implemented in the Early Access version. The roadmap provided by the developers suggests that it will appear after the 1.0 update or together with it. It is possible, however, that we will receive some elements of cooperation in the Early Access.