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".

Native of Georgia (US) here. We use "all y'all" as a possessive. I've never heard "alls y'all's" here, but have in the Southwest and thought it was linguistically slightly excessive. :-)

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).