Actually not exactly an office party - the head of our group is having a party at his house for us. He didn’t specifically say “and guest”. We are a fairly close-knit and friendly group - I think most of us know everyone’s spouses.
If people are bringing spouses, and you’ve been dating someone pretty exclusively for a few months, it sounds like it would be somewhere between “acceptable” and “expected” on both points.
I think if he didn’t say “and guest”, it’s pretty well understood that you should show up alone. Keep your ears peeled though, if he mentions anything about bringing a wife/girlfriend. It may be okay afterall. An exclusive girlfriend of a few months is invited if spouses are, as far as I’m concerned.
If the girl isn’t invited, tell her those were the rules set up by the host. Are you worried she’d be mad?
It sounds like you get on fairly well. Why not just ask? “Hey, Bob, are we supposed to bring our other halves to this?” doesn’t really create an expectation either way, does it?
If it’s a really serious relationship and other people are bringing spouses, you should bring her. But if you’re just casually dating or you still feel like the realthionship is new, I wouldn’t. She’ll probably be bored and the work parties are usually a chance for you and your coworkers to socialize outside of work. If you have to stay glued to her side while constantly introducing her, it kind of defeats the purpose of a work party.