In general, you can use lookup fields as you go from Grandparent to child (I kind of wish they were called lookdown fields, as they travel down the lineage...).
Then you can use summary fields to go to the reverse and go up the lineage. You can always summarize summary fields....just remember you may end up with a long text string by the time you get back to the Great Grandparent.
So if you want a community organization to show up on your Activity Table, lookup the field from Organization to Emergency Department to customer to Activity.
And conversely if you want all Activity info up on your organization, take a summary from Activity up to Contact....then another summary and another summary. In the end you will end up with string separated by semi colons. You can use SearchandReplace to change it into something more pleasing for the user to look at (perhaps line breaks etc).
