I've gone to company parties solo, I've gone with Partner to her's, I've gone as a "date" with friends, I've even gone with my mom on several occasions. It's never been much of a big deal in any of the offices.
I do caution, whomever you bring, make sure they can behave appropriately at a work function. If you bring them, they represent you.
I've met a lot of people's brothers, sisters, cousins, best friends, and even parents at work parties. I think nothing of it other then "Oh, this is someone Joe knows that is important to Joe, good to know", and I treat them the same as someone's husband, wife, or girlfriend/boyfriend. I don't think "Look at single Joe, bring his friend John, just to get John a free meal! Bad Joe!", the company gave him a +1, it's up to him to do what he'd like with it. Now if John gets wasted hops up on the table and gives his best Miley Cyrus impression...well now I'm thinking "Dang Joe, why'd you bring John? Reel him in!", but if John was Joe's husband, I'd be thinking the same thing.