My parents were pioneers in the "open adoption" movement, and as a result, I have close relationships to my birth parents (and even birth grandparents). Is this something to mention in my essay (as a "hook" or something admissions officers will even care about)?
Kudos to your parents (and to you) for maintaining close ties to your "extended family." I think that these relationships would make a great college-admissions essay topic. Your atypical situation will help you to stand out in a crowd, and it's also something that's important to you ... which is usually the best essay fodder.
The one piece of advice I'm going to toss out is that your essay may be most effective if you are careful to "show" the reader more than you "tell." And don't try to cover too much turf, either. Instead, perhaps you can focus on one specific episode (e.g., a dinner with both your birth parents and adoptive parents present or your thoughts in the car as you head off on a visit to your birth grandparents) and use this as the centerpiece of your essay and as a springboard for revealing more about your unusual family ties.
But, however, you decide to approach it, you definitely have some good material to work with, and your essay should provide admission folks with a helpful glimpse into your life that the rest of the application probably won't.