Jason, I suggest you start with a MAP. Look at roads, islands etc. I'm guessing your group will be a wide age range. I'm guessing you'll all be flying from the US. You will want a place you can get to readily from Athens -- that has a lot of culture, is on/near the water. Arrangement where various family groups within the larger group can have their privacy, set their own agendas.
I don't think a huge villa exists and besides not right for you. I suggest a complex that has a mix of hotel rooms, studios, bungalows. I also suggest looking at Tolo in the Peloponnese, a seaside village just outside Nafplio. Nafplio is a World Heritage Site, gorgeous & historic, great site-seeing, sunsets, chic shops. NOT an AMerican enclave (it's a fave weekend for athenians) Within 15-20 miles are some of the TOP stunning ancient sites of all -- Epidaurus, Nemea (#2 Games site next to Olympia!), Mycenae, TIryns. And Nafplio has THREE castles to explore. Tolo is a nice sandy beach, water sports, I think u can rent boats.
I have some specific Tolo places to suggest, not enough space here & would need to know more requirements, i.e., WHat Month? you can PM me.