Our 'Customer' for the opportunity entity is always an Account (the Customer not necessarily the subject of the sale for us). However, there also has to be a Contact on the Opportunity that is not necessarily related to the Account. To make this happen, I created a new field called ContactName that is a Lookup to the Contact entity.
If I create an Activity on the Opportunity where the Regarding field is the Opportunity Name the Activity automatically rolls up to the Customer (which in this case is an Account). How can I make the same Activity roll up to the Contact in my custom ContactName field so that when I open the contact from the Opportunity I can see that there was a (e.g.) Phone Call logged where Opportunity X was set as the Regarding?