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I go about it somewhat differently than @wdavis but end up with the same results
This workflow starts by finding how many members were added on each day, how many were subtracted and generating a list of unique days since the first. After joining and cleanup, the Multi-Row tool adds the current day's delta to the previous day's total. The bottom Browse just picks out the dates that you listed in your sample output
The numbers don't match your's, because your sample has one member joining In Nov 2018 so that person isn't a member in Sept 2018