Would I require to pre-book a taxi for 5 passengers in Beijing Capital Airport?

Well, first of all, most cabs in this city can only hold four passengers. Thus according to your situation, just hire a private mini van at the airport. In general, it takes around 1.5 hours for the single way to get to Badaling Great Wall. Maybe you can contact TravelChinaGuide in advance. They are reliable. And they can provide such tour route for you. If interested, the contact information is traveltravelchinaguide.com/tourstravelchinaguide.com. Hope helpful!