[SalesForce] How to configure OWD and Sharing sets to allow Customer Community user to see all cases for their account and any child accounts

I think the trick is similar to what one did for high volume portal users.

You can create a Sharing Set but instead of enabling access based on the Case Contact lookup, make the Case Account lookup the criteria driver.

Assuming your customer's colleagues are properly listed under the same Account, this would enable them to view each other's cases.

https://help.salesforce.com/HTViewHelpDoc?id=customer_portal_sharing_set.htm&language=en_US

As for doing this on an Account-by-Account basis, you would have two different Sharing Sets and two profiles. One you configure access by Contact lookup. The other has access by Account lookup.

Then you can assign different profiles to the users as appropriate.

Sharing sets are for the customer community (not plus), and other high-volume licenses where you can have millions of users but not "full" sharing rules.

CC+ has full sharing (rules, apex, and manual sharing), which allow way more than the more simplistic sharing sets. The trade off is that that user count can't go above 350k or so--otherwise the sharing calculation has the potential to get ridiculous.