Ok, so I watched the video you created on “Secondary Table Logic Inside of Power BI Using Dax.” I was able to retool the formula so that it worked for my circumstances, which was finding the individuals with registrations in the last 182 days (6 months).
Registrations Last 182 Days = CALCULATE(COUNTROWS(Registrations), FILTER( ALL(Dates ), Dates[Date] > [Last Date] - 182 && Dates[Date] <= [Last Date]) )
Now what I want to do is find the number of individuals without registrations in the last 182 days (6 months). Initially I started thinking this would require an additional filter to find those whose registration count is equal to 0. But thats where my thinking runs out on how to do this successfully.