working with a group of people.
Typically a bad idea, unless everyone in that group has proven by their historical behavior that they are incredibly dedicated and hard-working, and also everyone in the group has enough available time to work on the game.
dev-plan
You have it all upside down. Get something super basic running and then playtest it aggressively as you build, bit by bit. Like, start with moving a square around on the screen, then making it jump, then making it collide with other squares, then... and so on. Work on the feel of the game from day one, and download some free decentish art too. A game is no fun if it's no fun and you can't determine that without playing it. A lot. And changing it when it's not fun. No need for corpo powerpoints.
If you think this method sounds strange, remember that it's what Valve did back when they made games.