Here's an example of how the accounting would work after a new member joins a Squad that has completed a first purchase: suppose Squad members contributed 2ETH, 1ETH and 1ETH towards a purchase, then the % ownership would be mapped as 50%, 25% and 25%. If a new member comes in and contributes 1ETH, the new member's stake would be 20%, and the remaining members' stake would be 40%, 20% and 20%.