How to make a wall turn on in chat?

From a ServerScript

Use the PlayerAdded event to register when a player joins.

Use the Chatted event to register when a player speaks. Then check to see what they said was a command and go from there. string.lower transforms the string in to lowercase so the command will work whether it is uppercase or not.

local wall = game.Workspace:WaitForChild "Wall"

local players = game:GetService "Players"

function chatted(msg)
	if string.lower(msg) == "wallon" then
		wall.Transparency = 0
	elseif string.lower(msg) == "walloff" then
		wall.Transparency = 1
	end
end

game.Players.PlayerAdded:connect(function(player)
	player.Chatted:connect(chatted)
end)

For group only:

Use the :GetRankInGroup function on the Player to see if that player is a certain rank. For example,

game.Players.PlayerAdded:connect(function(player)
	if player:GetRankInGroup(0 --[[GroupId]]) == 0 --[[Rank Id]] then
		player.Chatted:connect(chatted)
	end
end)