I suppose there may be several ways to do this. This may be one:
Have a “date field” that is populated when a client is added (that would be a new record I presume, using a script step to test when a client is added then add the date or the month to the date field). Then have your “count field” count the “date field” with a given month search / find criteria. The Given month could be a pulldown list selection.
Thank you for your feedback. Little confused on how to go about this. Right now my date field is associated with any measurements taken. So the "date field" would be separate field all together?
You need a date field in the clients table as rouelf suggested. This field can auto-enter a creation date to automatically record when a Client first joins. For inactive clients that re-activate, you would update this field with the date they returned to active status.
You can also add a cMonthActive calculation field defined like this:
DateActive - Day ( DateActive ) + 1
This computes a date for the first day of the month that the client became active for all dates in that month. In a summary report, you can sort your records by cMonthActive to group them under/above/between sub summary layout parts "when sorted by" cMonthActive as a way to list all clients that became active in each month.
For more on summary reports, see: Creating Filemaker Pro summary reports--Tutorial
Dan, sure add a separate date field and or other fields or variables to get the job done. The summary reports link provided by PhilModJunk is most excellent.