Virtue of solidarity: identification of the good of others with whom one has substantial solidarity as a secondary good of one's own. Common cause meaning one acts to assist them as needed in tasks in life, at cost of one's own time/material prosperity/etc (opportunity cost). This goes beyond creating situations of mutual benefit (an art in itself). Problem: How does one decide how much solidarity to give, and to whom? General notion: virtue is not necessarily about self-benefit, especially in the short term. Problem: Virtue is a very hard sell, even for those with the best disposition, for those who feel their efforts are neither appreciated nor will be reciprocated - this kind of virtue needs to be "hooked into" one of those basic instincts on some level (even if that's not the point) to flow well with human nature. Essential prerequisites for high levels of this virtue: lack of excessive greed, self-identification with the greater societal good (note that this virtue is thus harmed by "values for capitalism", and has at least a moderate overlap with traditional/religious notions of virtue).
"Getting by" is also almost a neecessity in life - feeling that one has sufficient resources to live comfortably with some security makes it easier to help others in self-realisation. Solidarity of this sort might take many flavours, with one's "level of virtue" in each dependent on how comfortable one is with that area of one's life - "legal risk", "material prosperity", "time to do things" are three areas among many possible.
Conclusion: Societies where people have the resources to live comfortably and securely, and where greed is frowned upon are the best homes for growth of this virtue.