You want to be able to run a report to see what Accounts donated, and what its individual Employees donated, separately or together.
This is going to be a two part rollup: Rollup Opportunities to Contacts. Then, rollup Contact amounts up to Account.
First, create a custom Currency field on Contacts. It can be named something like Total_Closed_Won_Opportunities__c. Then create a currency field on Account called Total_Closed_Won_Opportunities_from_Contacts__c.
If you want a total of both Contact and Account donations, then create a formula field on the Account level called Total_Account_Closed_Won_Opportunities. You would add Total_Closed_Won_Opportunities_from_Contacts__c with the field that holds your Account-only donations.
With all of your donation information in one place, it makes it easier to talk with your donors while knowing the whole picture.
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