GLUT supports simple cascading pop-up menus. They are designed to let a user select various modes within a program. The functionality is simple and minimalistic and is meant to be that way. Do not mistake GLUT's pop-up menu facility with an attempt to create a full-featured user interface.
It is illegal to create or destroy menus, or change, add, or remove menu items while a menu (and any cascaded sub-menus) are in use (that is, popped up).