Well a couple things, if possible I call ahead and see if anything can be modified, and if not I find some reason to bail. If it's spontaneous, I benefit from, frankly, being a pretty heteronormative man so I can always manage to get beer and a large side. I really only have to let people know ahead of time if it's some kind of work event, in which case the staff member in charge of us attending the event handles it but I'm not in that situation a lot. Of course, most of the time I'm eating it with people who already know me well so they already know I'm vegan.