[bgdam]

My friends and I invented the 'Joint Venture' ruleset precisely because of this reason. Under these rules, you don't need to own a monopoly to start building on it. Instead you can form an agreement with the other owners, a JV so to say, and then you can pool money and start building. Income from the properties can be split up between the JV partners in any manner of their choosing.

But since the rules of the game only allow you to build if you have a monopoly, when a JV is formed, all owners should nominate one person and transfer their properties (within that monopoly) to that person, who then becomes the owner of the monopoly and can do with it as he wishes.

This leads to a lot of fun situations when there is conflict between the JV parties about the usage, or if the owner requires money for financing something else, and decides to 'backstab' the JV partners. Keeps the game entertaining for hours!