At least in this case, it sounds like you buy it upfront.
> Mondragon’s co-ops share the same detailed information with worker-owners, who buy into their co-ops by making one-time payments of roughly sixteen thousand euros in most co-ops.
I'm sure Mondragon's financing coöp will happily assist with funding a loan, underwritten by an insurance coöp and paid through the bank coöp. It's turtles^w coöps all the way down.