Sadly, it's not Keira Knightley to me. Hilariously and violently, it happens to be Deadpool!
(Click picture to for-better-or-worse-size)
I'm pretty sure you'll find pretty much anybody who's anybody in today's Marvel Universe at the wedding, from the Agents of Atlas to what little is left of X-Statix, from the first character who appeared in the M-U (Fin Fang Foom) to the newest (Shogo Lee). And, among Deadpool's groomsmen, star of comics and (soon-to-be-released) screen, Rocket Raccoon! And his KirbyTech wedding gun.
You know, R.R. could be the wedding photographer and that could be a camera, not a gun.
Naw, it's Rocket Racoon. It's a gun.
Edited on 03/09/14 to insert this addition:
Here's a peek at an upcoming Rocket Raccoon drink cup to promote Guardians of the Galaxy. It doesn't appear that he's holding a gun that especially Kirbyesque. Darn it.