Cooperative Charity

A Cooperative Charity is a useful structure for a user group, aggregating funds and allowing the users to deliberate before releasing them to fund specific UserStories.

The problem is, how do you create one? They're very different from corporate charities. You can understand them either as,

a cooperative which pays nothing in business taxes, and where membership expires after a preset length of time with no refund of the membership fee.

Or,

a non-profit where the board of governors is the entirety of donors, and donations have a floor of X dollars.

I have the sinking feeling that cooperative charities aren't recognized in commercial law. So how would one go about emulating it?

And how do you solve the problem of having to pay taxes on any leftover funds the charity hasn't released yet?