There are no restrictions for grouping cities/villages to join OrganiCity.
It is possible to get involved as several new cities in different ways. You can apply to run your experiment in these new cities. In this case, you should carefully consider the feasibility of such a project among other things. For example, establish necessary contacts (e.g. with the local authorities) and choose a few cities out of the “parco” to start with (as experimenting in 12 cities at once is very resource-consuming). You can plan to develop your experiment in other cities/villages of the region beyond the initial experimentation phase.
One more way to get involved is to enable other people and organisations in the cities to submit applications to OrganiCity by hosting events to help generate ideas and network. From a technical point of view, the latter can include (but does not have to) setting up OrganiCity server and federation of the cities' data on this platform.