The members of the development co-op are the people on the wait list.
The co-op determines where and what to build.
There is an escrow account that holds the member loan money.
Money is deposited in a low risk investment vehicle.
Returns are deposited in the escrow account.
No lock in, may leave at any time, access to money is by the rules of the investment vehicle.
There is also a member loan, in general, this will be returned to people leaving the co-op (either by joining a housing co-op or leaving the wait list). However, this is not guaranteed.