Anyone is able to create Ideas. Simply click on the "New Idea" button on the Ideas page:
However, in premium Independent Projects or premium Team Projects, Admin(s) have the ability to limit who can see the Ideas feature (either, Everybody, Supervisors or Admin(s) only).
Also, in Premium Projects, Admin(s) can restrict who can create and edit Idea categories.
To see all benefits of upgrading your Project to Premium, click here.