My first suggestion is that many people dress hideously for hen nights so you need not worry. Anything is better then a nun/sexy bride/sexy kitten outfit!
Gold lace cardigan/jeans/ white T shirt, a bright lipstick, some eye make up and if you have a necklace or bracelet and earrings if you wear them, that will look good. i would wear a pair of heels or some flat summer sandals. Any will do. If you have perfume a good spritz or have a squirt from a store tester on your way home from work/town! Many stores will give little samples out too.
Also consider a clothes borrow/swap with friends. A pair of shoes, a borrowed scarf, necklace can make you feel great.
Does your DH/partner have a plain white formal shirt? You could wear that with a jean or black pant, heels or boots with a bright lipstick and you'll look seriously cool in that Emmanuelle Alt (editor French Vogue) way. here is a picture of her in a black jean/pant and t shirt look with black court shoes. A black shirt from DH would work too. Plenty of charity shops have amazing quality black/white shirts for sale. i would buy them. Worn with a big necklace, lippie and blowdried hair, simple is the best.
If you have a belt to put on with the shirt/T/jean look even better.
I very rarely 'dress up'. I'm not fond of the super glamorous dress and heels look. Most of the time, simple is best. And a big smile is best of all.
Your friends want to be with you....