So, whenever you join a Roblox game, a message appears in your chat saying "Chat /? or /help for a list of chat commands.", but how/where would you change that?
If you test in studio, you can get all the roblox chat scripts from the chat service, then copy and paste into your game. Enter the ChatModules and go into the ChatCommandsTeller module, around line 59 you will see where you can change the default welcome message!
Alright so that is the default ROBLOX chat system, you can disable it by going into PlayMode and Copying all scripts inside of Chat then exiting play mode and pasting it back into chat. Then you can figure everything from there..