Adding an Event Category to a project means that when an event is created within that project, the user creating the event will be prompted to select whether the event creation form that is used is the one associated with the Event Category that you have added to that project
Before you can add an Event Category to a project, you must first turn on Events within that project: Enable events on a project
Steps
Go to the Project Preferences page of the project that you want to add the Event Category to
In the Event Categories assigned to this project field, add the names of the Event Categories that you want to be available when people are creating events on this project
Press Save