How would I modify this script to change a leaderstat according to their rank in a specific group?

function onPlayerEntered(newPlayer)
	local stats = Instance.new("IntValue")
	stats.Name = "leaderstats"

	local c = Instance.new("IntValue")
	c.Name = "Access Level"
	c.Value = 1

	c.Parent = stats

	stats.Parent = newPlayer 

	newPlayer.Changed:connect(function (property)
		if (property == "Character") then
			onPlayerRespawned(newPlayer)
		end
	end)
end

game.Players.ChildAdded:connect(onPlayerEntered)

I need this for my technology facility, where only certain ranks can enter certain areas.

You're looking for the GetRankInGroup method!

function onPlayerEntered(newPlayer)
    local stats = Instance.new("IntValue")
    stats.Name = "leaderstats"

    local c = Instance.new("IntValue")
    c.Name = "Access Level"

	local rank = newPlayer:GetRankInGroup(0) --Change this to the Group's ID
	if not rank or rank <= 50 then
	    c.Value = 0
	elseif rank <= 100 then
		c.Value = 1
	elseif rank <= 200 then
		c.Value = 2
	else --highest ranks
		c.Value = 3
	end

    c.Parent = stats

    stats.Parent = newPlayer 

    newPlayer.Changed:connect(function (property)
        if (property == "Character") then
            onPlayerRespawned(newPlayer)
        end
    end)
end

game.Players.ChildAdded:connect(onPlayerEntered)