It's organised by Bringstuff who say "We want you to come dressed as a visual interpretation of a Tube station - a blacksmith with a large hammer for Hammersmith, a manchester clown for Piccadilly Circus or James Bond for Bond Street etc"
"You get the idea, so be as creative as you can, with one exception. NO ANGEL COSTUMES - Angel Islington is off limits as it would be rubbish if everyone turned up in the same costume. "
Full details are on their website & entry's free before 10pm. If you're looking for ideas for Tube Fancy Dress party costumes, there's lots in the comments of the aforementioned post, or you might want to suggest some of your own.
I might be tempted to go along covered in mouldy ham representing a station I change at a lot on the District Line. Although that's a tad unsociable, so I'd probably dig out an old duffle coat, grab a small suitcase & a jar of marmalade and learn Peruvian for the evening.