I've been playing minecraft for a while and one thing i know is, let your imagination go wild! When i build cities villages etc. i plan ahead, usually i have a pen and paper besides me and i start making shapes, i make sure i apply a little of architecture it really doesn't take a guru to build in minecraft. One tip from me is you need to explore go outside visit cities and look at the buildings, houses, and focus on the shapes on the design it really helps you out alot. One thing you need to keep in mind is to measure and have a symmetrical idea of what you're building. Look at photos and apply your own designs or watch videos on youtube for inspiration. I'm new at building in minecraft but i been building in real life since i was a kid.
Builds with that many people are hard, but you don't actually need many amazing builders to do that. Basically, you just need a someone to organize and lead the build, and then five to ten core builders. After that the rest of the builders just need to adapt to the style and make sure everything fits in with the theme. It also helps to all be in communication while building though.
I think it would work best by taking a few of the best builders in the group and making them team leaders. Plan out a style, design, and rough blueprint so everyone is on the same page. Then put them into small teams with other players and give them a piece of the overall project to build. Go around and make some small changes that you think would benefit the whole build, since you will be able to view all of it. Take some short breaks every now and then to get a whole picture of the build and make any necessary changes as you see fit. The just keep on going I guess.
It's the same concept as running a business. Just delegate tasks to minimize the amount of building that you'd need to do. This would also bring in other styles or suggestions that you hadn't thought of. The team leaders would also in turn be able to delegate even small tasks to their group such as having one person build a tower while another works on a roof design. Before you know it you will have a fairly large build hopefully. I've never done anything like this so I honestly have no clue how it works. Obviously the longer you work with this group the better the builds will be as they all will learn and grow with each other.