Hi Erik, are you looking for distinct users of the project or the distinct users that are logging in every day ? The first one shouldn't frequently change unless you are adding new users or deleting them. In the second case although it depends on the history tables. Also , here is a great post which has shareable data sources that can answer most questions about your server.
I'm looking at distinct users at the project level. The only fields on my viz are the project folder name, the month of LOGIN AT (date field), a filter on LOGIN AT to scale down to the last two full months, and the number of associated users (a COUNTD on FRIENDLY NAME).
And my COUNTD on FRIENDLY NAME actually drops slightly on a day-to-day basis (typically, anyway - sometimes, they are spot-on).
That's why I'm wondering if workbooks being deleted from within that project folder could be causing this slight decrease. Or perhaps workbooks being promoted to the project folder could somehow be "resetting" the numbers on a day-to-day basis.
I'm also wondering if things like workbook refreshes could somehow be coming into play here.