It's for a friend's 30th/hen do. I haven't seen her or any of the others in about five years and I've aged badly in that time! 
We have been told to wear dresses and heels. Not my thing at all. I'm hoping to wear a smart pair of flat shoes with a dress but have no idea what kind of dress or shoes to go for. I could possibly cope with a very low heel but cannot walk in high ones.
They are a glamorous bunch and I think they'll put a lot of effort in. I don't want to look too OTT in case I'm wrong but equally I don't want to look underdressed, so would like something that will blend in with everyone whether they have gone super glam or more casual. Is that impossible?