Navigate to the Requirements tab in a project's primary horizontal menu. Click the New button in the top right corner and select Requirement from the dropdown. Add a title in the pop-up and click Save, or Save & New to create another requirement.
A user must have view and edit permissions in requirements to create requirements. Depending on the settings of the user's project permissions, they may be able to view all the project requirements or just a subset of them. The same is true for viewing the list of project members when adding project contributors and reviewers. Users added to a specific requirement as a contributor or reviewer will be able to view it.