Could someone help me brainstorm this? Getting text after a space?

Here is a way of using string pattern to turn each string after a space into a table.

-- The string we want to test this on
local str = ":pm PlayerName [MESSAGE HERE]";

-- The "arguments" table
local tab = {};

-- Starting the string after the ":" (the prefix) so it doesn't get included
str = str:sub(2)

-- Getting each text after whitespace (which %S+ is for)
for w in string.gmatch(str, "%S+") do
    -- Inserting each word into the arguments table "tab"
    table.insert(tab, w)
end

-- The command
local command = tab[1];
-- The playerName
local playerName = tab[2];
-- The message to send
local message = table.concat(tab, " ", 3)

print("Wants to send", playerName, "a message:", message)

This will of course require you to do some modifications, but it is a basic example of how to accomplish something like this. I recommend you check out the following articles:

String patterns: http://wiki-origin.roblox.com/index.php?title=String_pattern String patterns reference: https://developer.roblox.com/articles/string-patterns-reference