Is "ground fighting" another reason we train kokyu dosa and suwari waza in Aikido? It just occured to me reading this thread that if someone had me on the ground, I would instinctively get first on to my knees. "Duh! That's suwari waza!"
I've never had anyone try to grapple with me in a serious fashion. Brother's are good for rough-housing but nothing serious. I should think if you can avoid the pin, despite being taken to the ground, you should be able to manuver and start some suawari waza and pin their butts with ease... granted you've trained enough in it.