Please explain how to respond to only the sender of a group message; and how to tell who your reply was sent to

I am very unclear about how the group message feature works. First of all, will I receive group message if I have MMS and group messaging disabled?

Also, how do I know it's a group message when it comes in before I reply?

How can I be sure that I am eventually only texting the original sender in the future; how long do replies go to everyone initially in the group message?