How do I create an API to retrieve information from the ROBLOX Website?

For example, I want to know how many members a group has and which of these members are onine.

  • I'd have to let the game server send a request to my website (I know how this works)
  • I'd have to let my website look what members are part of the group. (However I also heard ROBLOX has an API for this already)
  • I'd have to let my website check which members are currently online. (I have no idea how to do this)

I already have a web server and basic knowledge of PHP.

Anyone can help me out on this?