How would I detect player movment inside a part?

I want to know how to detect player movement inside a part, but the only way I know how to do this all the players will get detected if one player move's, I would like to know if a player were to walk into a part with multiple people only that person will get detected.

I am half asleep so I have no idea if this will work with multiple people.

So I put a server script inside of the desired part and a folder called PlayersOnPart

The script inside of the part

local Players = game:GetService("Players")
local Main = script.Parent

local Range = 40
local Check = true
local stop = 15


	local function balls()
	for _,Player in next, Players:GetPlayers() do
		local char = Player
		local name = char.Name
		if script.Parent.PlayersOnPart:FindFirstChild(name)then
			wait()
		elseif not script.Parent.PlayersOnPart:FindFirstChild(name) then
			local namedong = Instance.new("BoolValue",script.Parent.PlayersOnPart)
			namedong.Name = name
		end
		end
	end

local function noballs()
	for _,Player in next, Players:GetPlayers() do
		local char = Player
		local name = char.Name
		if script.Parent.PlayersOnPart:FindFirstChild(name) then
			script.Parent.PlayersOnPart:FindFirstChild(name):Remove()
		end
	end
	
end
	while wait() and Check do
		for _,Player in next, Players:GetPlayers() do
			local Char = Player.Character or Player.CharacterAdded:wait() 
			local HumanoidRootPart = Char:FindFirstChild('HumanoidRootPart')
			local A,B = HumanoidRootPart.Position, Main.Position

			local Dist = (A - B).Magnitude
			if stop >= Dist then
				balls()
			else
				noballs()			
		end
	end
end

if its confusing or if it doesn't work tell me tomorrow cause I'm so tired.

There is also a script inside of the workspace that gets the amount of players, I made it so it named the players inside of the part so if you wanna do anything with that go ahead, but here is the script for now.

local Folder = script.Parent.Part3.PlayersOnPart

while true do
	wait(1)
	print(#Folder:GetChildren())
end
	

Next time you ask a question try doing it yourself first or show some code of what you tried to do. Anyways gl with you game