Add a rule “Groups” with the following wording:

If a Player owns two or more Squares with consecutive numbers, and those Squares have at least one Colour (except Beige) in common, that player may choose to make those Squares into a Group, at a cost of B$5 per Square..  This new Group may then be named by the Player.  No two Groups may have the same name.

Squares may be added to a Group at any time, unless another Rule prohibits it, for the same cost.

Squares may be removed from a Group at any time unless another Rule prohibits it.  This action refunds B$3.

Groups are tracked in the World, next to the Square listing.

Add the following text to rule 13.7.1 “Rent”:

If the Square is part of a Group, the rent is multiplied by the number of Squares in the Group.

And before someone complains about there being no colour green in the ruleset: Add the colour Green, with code Gn, to rule 13.1 “Colour Coding”.