Enabling Developer Console for only a certain list of people?

You can't check if something is part of a table like that. You'll have to write a function that loops through the table, and matches any values with those inside the table.

It'll look something like this:

local function FindValueInTable(tbl, val)
	for _,v in next, tbl do
		if v == val then
			return true
		end
	end
	return false
end

Also you'll need to put a loop on making the DeveloperConsole visible.

local plr = game.Players.LocalPlayer
local gui = game.StarterGui

local whitelist = {
	"AlreadyPro"
}

local function FindValueInTable(tbl, val)
	for _,v in next, tbl do
		if v == val then
			return true
		end
	end
	return false
end

game:GetService("RunService").RenderStepped:Connect(function()
	if FindValueInTable(whitelist, plr.Name) then
		return
	elseif gui:GetCore("DeveloperConsoleVisible") then
		gui:SetCore("DeveloperConsoleVisible", false)
	end
end)