Actually, you don't have to have an i-product anymore to deregister from iMessage. However, this method doesn't always fix the issue. You can go to this
link at apple in order to deregister from iMessage if you came from an iPhone.
Like I said, though, that may not be the only issue. Each group message you're currently in will need to be deleted by anybody who has an iPhone and start a new message. That doesn't always make it work perfectly, either. The other thing I've seen mentioned is to have each and every person whom you're a contact for go into their contacts app on their iPhones and change your number from "iPhone" to "mobile" or some other tag/label.
I still don't get all my group messages, so I don't think there's one "magic bullet" to fix this stupid issue.