LayerTree
Lightning Labs published a design that lets thousands or millions of people share one Bitcoin output. Balances sit in an off-chain Merkle-sum tree; covenants keep the operator from taking funds, and any member can still leave alone in a single transaction whose size doesn’t grow with the group. It needs opcodes that have been proposed but aren’t active yet, so it’s research with a reference implementation rather than something you can run today.

https://layertree.tech/