Kevin, my interpretation is the OP is a married poly woman, and one of her other partners (not her husband/wife) has asked her to the event.
If this is the case, I see a few options:
-If she's not ready to be "out," or if they aren't ready to be "out," then declining is probably the best option. Lying never goes well for anyone.
-If they are ready to be out, then a simple answer is, IMHO, always best. Something like "this is my GF, Koshka." And just leave it there unless someone asks. If no one does, don't offer. If someone does ask about the ring, just be open but short: "My husband and I have an open relationship."
No one is really entitled to the details of your private life, but often they ask anyway. I'm a private person, and don't really talk about my relationships with coworkers and the like. I've found keeping it simple and very direct, as well as without personal details, is best, and staves off all but the rudest people (to them I just say "I'm sorry, but that's really none of your business").