My aunty did this on her birthday, i was her bridesmaid and i didn't even know it
She told everyone it was a dress-up party (based around the yr she was born), we helped set up and then she was like come on we are going... WHAT WHERE.. off to get hair and make-up done.. your my bridesmaid

It was gorgeous, the people that REALLY wanted to be a part of her life and celebrate things with her were there, a special few who said they couldn't make it but really needed to know were told and they cancelled previous commitments but otherwise it was a total surprise.. good luck with ti all