There are many ways to tackle problems like this, ranging from very simple to extremely complex, depending on refined requirements.
If I understand the problem correctly, you just want to count for each day how many users have signed up and how many cancelled, and then calculate the difference. If your data is in Excel, then the simplest way to do it is to re-shape the data using Tableau's Excel add-in (as explained here: http://kb.tableausoftware.com/articles/knowledgebase/addin-reshaping-data-excel) and do a few simple calculated fields. I attached the spreadsheet with reshaped data and the workbook with a solution.
If, however, you want to know how many active users there were on a given day, (i.e. how many signed up on that date or before and haven't cancelled yet), and/or your data cannot be reshaped for whatever reason, then this requires data padding/densification and other acrobatic maneuvers that can get quite complex.
This worked perfectly, thanks!