Jimandy, I am going to guess you are new to UE4 from your very first post, and new to game development from what you wrote this thread. Your question, the way it was asked, with the limited info attached, is a question pretty much only a person new to game development would ask. (I can expand on this if anyone asks why I would say that) Its perfectly fine to be new to game development. I am new as well, in fact.
The reason why I mention the fact that you are likely new to game development is because your game idea is HUGE. If you are talking about GTA Five, you are talking about DECADES of work, if its split among 10 people. Now, maybe you mean without a lot of the content in that game. Do you want five main characters with different voice actors? Do you want a massive variety in the types of cars, and NPCs roaming around? How many voice actors are you going to get? Do you want the radio stations in cars with a huge amount of just extra content to listen to? If you can live without those things with your group of 10 developers, you’ll shave years off your time, but that still doesn’t make it feasible likely.
So to give you more advice, answer these questions:
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How much experience do you have making games? - If the answer is “very little” then the advice you are going to get from every person here is going to be “start MUCH smaller”.
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Describe your team of 10 in more detail. Are they already all onboard? How knowledgeable are they? - I will tell you that probably everyone here with a fair amount of game development experience is guessing that this team is NOT a cohesive group of developers experienced and motivated enough to stay together long enough to come close to making a game like you are suggesting. I would further guess most people here are suspicious that the team een currently exists at this stage.
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Describe some of the major features of the game that will affect development time: Multiplayer? Voice Actors? Sand Box game? Single player story? - Again, if you are new to game development, the answer will be to start much smaller.