You cannot 'associate' text with geometry at its insertion point, through the API. It has long been asked for... There are a couple of workarounds... Make it as 'flat' 3d-text [or normal Text] - make it inside a component definition, which has is2d gluing behavior, add_instance of it at the face.bounds.center point, then use instance.glued_to=face so it moves with the face...

@nicoiweins said: OK it seems great and for a bounding box? use group_instance.bounds() which is inherited from Sketchup::Drawingelement, the superclass of Sketchup::Group (and most all model objects that are seen.) Even more model objects are descended from Sketchup::Entity, including Sketchup::Drawingelement. Each Ruby class inherits from it's ancestor classes.