One Background, usually Domain, is chosen as an “anchor” for the others, and then every player who wishes to can contribute more backgrounds to the shared pool. Note these on your character sheet somehow, since anyone might tax them and they may not always be available.
For example, the Warehouse is a 2pt Domain currently, and other backgrounds added to the pool will improve the Warehouse, and be connected to it, but available to any PC who has contributed to the pool. That is, I can contribute a dot of Retainer to the pool, but then later draw on a point of Resources that someone else contributed.
These are not cumulative. For example, if someone contributes 1pt of Resources to the current Domain, that doesn’t benefit Thaddeus who already has a Resources score of 5, but it would benefit anyone with a 0 or 1 (since it would give them a 1, or essentially twice what a 1 normally provides, so they can be used/‘taxed’ twice in a month)