Context: I've been Primal for 4-5 months or so. I think that context matters.*
I don't eat grains ever when it's just me. If I have friends over, they eat what I cook. Likewise, when I'm over at a friend's place, I eat what they cook. Now, I personally have no allergies or serious issues to work around. If I did, I'd ensure those were known to friends and family well in advance of eating over/going out and operate accordingly.
* If I was still in my first few weeks of transitioning to Primal living, I'd opt out of "social eating" simply because small set-backs can wreak havoc with some people, either biochemically or just plain guilt-wise. I'd simply tell people what was up and hope they don't feel bad about me not being there for the meals themselves, which is to say there's nothing to stop you from going over after!