You can wear whatever you want, but remember: This is the office party. This is a72nd Birthday Turning 72 Years Old Shirt tee of people with whom you work, so if you wouldn’t wear a revealing dress to work, don’t wear it to the office party. Also, don’t drink much you presumably know your limit, so stop well short of it. Because again—you work with these people. When I worked at TV Guide, senior staff regularly attended the Christmas parties, which (at least at the beginning) were lavish, usually held in off-site venues and allowed employees to bring spouses. You don’t want your boss’s boss asking who that was—the girl in the thigh-high bandage dress and hooker heels or the guy who threw up on the white-glitter sparkle Christmas tree. Women get the brunt of the judgmental post-party gossip about attire while men generally have to do something memorably bad, but I imagine a male manager showing up in gold lame hot pants would cause a stir in most business environments.
So I went back to find the72nd Birthday Turning 72 Years Old Shirt tee where Ron agrees to go to the party with Hermione “just as friends” and in the paragraphs following the greenhouse scene we’re told that, “Harry watched his two friends closely over the next few days,” and that they were a bit politer to each other than usual. Harry supposes that he would “just have to wait to see what happened under the influence of Butterbeer” at Sluggy’s party. This seems to imply that Ron in fact, did agree to go to the party with Hermione, and that she very likely said “just as friends”… which is a perfectly appropriate thing to say when you’re just trying to see how things might work out together. Frankly it just makes Ron’s behaviour and deluded justifications even worse if he had agreed to go to the party with Hermione and then snogged Lavender Brown.