If the group being spoken to is exceptionally large, or it's clearly a group made of smaller groups (a group of groups), in some places "Alls y'all" is used. It's fascinating to note it when you hear it.

edit also, if there's a possessive it gets pretty crazy, for example a tour guide, assembling several groups for a tour might say "Let's get together for alls y'all's tour".

Oklahoman here. We say "y'all" as plural. We also say "you all."

Y'all is most certainly plural where I'm from (Southeast US).