The "blue bubble" just signifies someone's using the iMessage platform, and we still have multiple messaging platforms nowadays. Think of it like "Usenet friends", "IRC friends", "ICQ friends", "AIM friends", etc.
I don't know you, of course we're not friends. I addressed this in my comment. There are group chats you are part of and ones you won't be part of. Most people maintain multiple different overlapping group chats.