A lot of people work Griever into to R=U theory. The fact that Squall gives Rinoa his Griever ring and that Ultimecia uses the Griever as a GF later convinces many that Rinoa/Ultimecia summoned Griever from the ring much like Doomtrain is summoned out of a ring.
But I gave up this argument a long time ago. I don't even know what I side I was on.