In the Bugs tab of the project's primary horizontal menu, click on the New Bug button. Enter a bug title and press return/enter or click on the Save button to save a bug. You can also click on Save & New button to create more bugs.
Users will need view and edit permissions in bugs to perform this action. Depending on the settings of the user's project permissions, they may be able to view all project bugs or just a subset of them. This is also true for the list of project members for whom to assign the bug. Watchers specifically added to the bug will be able to view it.