How to make the door open with one chat?

You need to use the Chatted function. When you call the function, you will listen for the message, "-opendoor".

You will also need to use string.sub, and string.lower.

local player=game.Players.LocalPlayer --getting the player
local door=workspace.Door --this is where the door is

player.Chatted:connect(function(msg) --when the player chats something, listen.
    if string.sub(string.lower(msg),1,9)==string.lower("-opendoor") then --if they chat "-opendoor" then.. using string.lower is better because it isn't case sensitive.
        door.Transparency=.8 --Makes the door semi-transparent
        door.CanCollide=false --You can walk through
        wait(2) --after 2 seconds
        door.Transparency=1 --Makes the door opaque
        door.CanCollide=true --You can not walk through
    end
end)

Only allowing certain players to say "-opendoor"

local player=game.Players.LocalPlayer --getting the player
local door=workspace.Door --this is where the door is
--v add people you want to be able to say "-opendoor"
local allowed={
    ["Megadude1536"]=true;
    ["SimplyRekt"]=true;
    ["Friend"]=true;
}

player.Chatted:connect(function(msg) --when the player chats something, listen.
    for i,v in pairs(allowed) do
        if v.Name==player.Name then --if their name is in the allowed list then
            if string.sub(string.lower(msg),1,9)==string.lower("-opendoor") then --if they chat "-opendoor" then.. using string.lower is better because it isn't case sensitive.
                door.Transparency=.8 --Makes the door semi-transparent
                door.CanCollide=false --You can walk through
                wait(2) --after 2 seconds
                door.Transparency=1 --Makes the door opaque
                door.CanCollide=true --You can not walk through
            end
        end
    end
end)

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