I just had to go to an AHR this weekend. It was a friend of my FIs who he is not super close with(which is why we didnt attend the DW). Of course I kept asking him to see what the attire is but he never did! We had NO clue what to wear. I looked on their website and invitation - nothing. Coming from someone who just dealt with going to one I would say it would have been very helpful to read on the website what is okay. Most guest were fine & casually dressy. My FI wanted to wear nice kakhi shorts and a polo. It was 95 out and he refused to wear pants. he was so nervous that he would be the only one not wearing dress pants. We got there and people were in pants, nice dress shorts and some even in suits. It was definitely a mix but everyone looked good and no one worse hanging out bbq clothes(if you know what i mean!) The bride, groom & bridal party all wore their same attire for the wedding. But honestly, the people in suits were probably dying and looked a little silly in the back yard.... Just my opinion!