I picked up a tiered chiffon dress from WhiteHouse BlackMarket over the weekend for my employee's wedding that I think would work great for any office party. It has a high neckline and is modest, without being too stuffy. The dress is cut on the smaller side so most reviewers have suggested ordering up a size, but I found the 00 to be about right for me. My only gripe is the length but since I got it on sale ($79.99!) I don't mind paying extra to have it hemmed.
http://origin-www.whitehouseblackmarket ... RRWidgetIDCheers,
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