How can I set it so only certain members in my group can wear this specific shirt?

wait()
local Shirt = "http://www.roblox.com/asset/?id=981314839"

game.Players:PlayerAdded:Connect(function(Player)
	
	if Player:GetRankInGroup() >= 246 then
		local news=Instance.new("Shirt",Player)
		news.ShirtTemplate="http://www.roblox.com/asset/?id=%22..Shirt - 1"
	end
end)

Your variable is already formatted in a way it can be directly inserted inside a decal/shirt

Also, you may want to use player.CharacterAdded so that each time the player respawns, you clone the shirt to him

wait()
local Shirt = "http://www.roblox.com/asset/?id=981314839"

game.Players:PlayerAdded:Connect(function(Player)
    Player.CharacterAdded:Connect(function(char)
        if Player:GetRankInGroup() >= 246 then
            local news=Instance.new("Shirt",char)
            news.ShirtTemplate= Shirt
        end
    end
end)