It is unfortunate that the invitation is formal. This tells the guests how to dress.
Your bridal party can wear informal clothes. Just make sure your guests know what to expect. So, it would be best if you could spread the word of the informality. Please do this soon.
I just realized that you could mean that you received an invitation, not sent one. So, you could be a guest. This changes things.
Today, people are mixing things up. We can't assume that an outdoor backyard wedding is informal anymore. This could very well be on the formal side. More than likely it would be nice dresses and suits. If it is during the evening, the little black dress or its equivalent would be best for the ladies.
If it is at all possible, it would be great if you could contact someone close to the couple to find out how formal this is. For the men to get out of wearing a tie would be great.
Should Wedding Guest Wear a Suit for Outdoor/Backyard Wedding
