The word bridesmaids is a common plural noun. It requires no apostrophe.
The 10 bridesmaids tried on their dresses.
If the word bridesmaids has a possession or belonging, it needs an apostrophe.
The bridesmaids' luncheon is at 1 o'clock.
The bridesmaids' dresses were ready.
The singular possessive is bridesmaid's.
The youngest bridesmaid's shoes did not match the other bridesmaids' shoes.