Ok all you people who just can't wait! Below we will list ANY spoilers that we get a hold of. But if you've entered here - don't blame us for ruining anything! Enjoy!
Inside sources state: "Michael's MOTHER is coming to the dinner. That's the woman with the brown hair & bun in the corner of one of the pictures. Who is she? BETH GRANT! She played Kelly MacDonald's mother in No Country For Old Men."
Ok, another theory I've been told by another source is "It [will] be Dwight's mother that he brings as his date." Also played by Beth Grant.
So, that's the thing with spoilers, even the people in the know, don't always know. :)