Doing open world game without planning out each level on paper (or photoshop or any other software) is a huge pain the neck.
Draw a map, split it into chunks (like Bethesda does) and work on each chunk separately. Also, I’m pretty sure you can send other chunks to the other members of the team.
Like people mentioned earlier you can stream levels. Basically it will be visible from distance, but unplayable. My advice, do something smaller. Do one location, which is surrounded by something, mountains or a barrier. And then you will see how quicly and how complicated things will get.
infinity.
depends on the detail, size, materials, meshes … etc etc. 32 gee-bees is good. start there. 16 will hurt a little. you should be ok with 16 if you are only doing 1 or 2 chunks, depending whats on them.
Honestly - by the sounds of it, you need to learn Unreal before jumping to something like multiplayer games. A simple single player based from other things from the “learn” section will do you right. I started working with Unreal (the game) and then later moved to mods for ARK … and now im on my own projects. Its been a less-then-exciting path, but, i cant imagine starting with the biggest, most complicated task this engine (or any) has to offer.
I cant imagine coming to a game engine and saying “guys, i want to make something like world of warcraft, can i do it with 3 people and 0 experience?”. OMG my head would explode lol … much … like … your teams will when your about 2 years into it and about 10% done … not trying to be a downer here, but, its seriously a massive task.
Also the cost is insane. If you dont have 1 solid mil to drop, dont think about it. Lets even say, on the simple side, if you dont have 50k to toss in the toilet, dont bother with open-world / multiplayer. Im not kidding. Unless your team can make 100% of its own assets, code, blueprints, network solutions, etc etc, and, is ok taking 0 pay for at least a year … then … then you maybe ok lol.
He’s correct, I’m a single-man team hobbyist, trying to achieve a open world of say, 4x4km floating islands procedurally. I’ve been messing around with it for about a year and a half now, and I’m not far yet… but, it’s a hobby for me in my spare time 
Reseach how many people a team like Bohemia Interactive has to work on the Arma series. Now do this x 40-60 hours a week. Then do it for you and your friends until you reach the same time. Multiply by 3-10 times for no experiece…