Constant values in Pivot Table

I have an issue that I can't resolve.

I'm creating a combined analysis/pivot table where the customer wants to see (by Calendar Month/Year) the total number of Completed Activities, a count of Accounts with Completed Activities, and a count of ALL Accounts that are owned by a specific User. 

In the Calendar Month/Year prompt they want to be allowed to select as few one and as many as 10 values.

My problem is getting the total number of Accounts into my report.  If I include the Calendar Month/Year and the Nbr of Activities from the Activity object, my count only includes Accounts that have related Activities.  I can get the total number of Accounts in a separate Analysis without the Month/Year.  But I don't know how to merge each Account/Activity entry with the entry from Account.