Distinct Count Based on Filter Expressions

ChumaAmako - I would try this way:

No of Active Users =
  VAR __MaxDate = MAXX(ALL('Order Table'),[Order Start Date])
  VAR __MinDate = __MaxDate - 28
  VAR __Table = FILTER(ALL('Order Table'),[Order Start Date]>=__MinDate)
RETURN
  COUNTROWS(DISTINCT(SELECTCOLUMNS(__Table,"Email",[User Email])))

Hi ChumaAmako ，

Create 2 measures as below：

_Count active = CALCULATE(COUNT('Accounts Table'[Email Address]),FILTER(ALL('Accounts Table'),'Accounts Table'[Email Address]=MAX('Accounts Table'[Email Address])&&'Accounts Table'[Days Since Last Purchase]<=28))

_No of Active Users = 

CALCULATE(DISTINCTCOUNT('Accounts Table'[Email Address]),FILTER('Accounts Table','Accounts Table'[_Count active]>=1)
)

And you will see:

For the related .pbix file,pls see attached.