(Note: I realize the original event in question is in the past, but we are entering prime wedding season, so it seems like a good topic to discuss.)
I think asking the person who invited you is fine, but do it early. My bridezilla moment came when in the 48 hours before my wedding, I started getting phone calls and texts asking me for info that was not only on the invite, but also on the website I had created just for guest’s convenience. At the rehearsal dinner, I told my sisters/ bridesmaids that I didn’t want to get any phone calls that night unless someone was sick or there was an emergency. (Most of my guests were family, and they were the ones who didn’t bother to read.)
As far as dress code, my wedding was nice but not fancy. However, I was dismayed when my stepmom said she was going to get the kids (my nieces and nephews) matching U of F t-shirts to wear. Umm, go Gators, but since neither my spouse nor I attended U of F, I thought that a bit weird and inappropriate. And unless the wedding is ON the beach, your outfit probably shouldn’t be on par with something worn at the beach, or to wash the dog. (Luckily, my siblings stepped in, and the kids all looked great in khakis and short sleeve button downs/ dresses, etc. I didn’t want super fancy, but was hoping for maybe the equivalent of school picture day, and was not disappointed since my siblings recognized that my wedding was not a football game!)
Now, I HAVE been to a wedding where everyone wore shorts and it was fine. It was a picnic and we had barbecue take out, so it was definitely a casual event. It was my uncle’s second marriage and I was just a child, so I don’t know what the invitations indicated, but I’m pretty sure that everyone knew it was going to be super casual. (The bride and groom may have been injeans?)
I think when in doubt, it’s better to err on the side of too dressy than too casual, as long as you don’t have a medical condition that makes dressing up unwise for some reason. The trick, I think, is to find clothes that LOOK dressy but feel comfortable.
Re the backyard wedding, are there any indications re fanciness level? Some ritzy people get those fancy white tents, etc. Or it could be a slightly nicer version of a cookout. I definitely agree with the previous posters … be careful re your shoe choice!