Help needed graphing a count between a date range

Not fully, no.  This is still an open item.

I just wanted to let you know that I have just opened a similar query about the last part of your question: how to expand by month instead of by day. It's located here:

http://community.powerbi.com/t5/Desktop/Column-formula-to-expand-a-date-range-by-month-instead-of-by-day/m-p/414995#M190505

If you've figured out a way to do what you needed without the full expansion, please let me know.

I was able to figure this out, details are on this thread.

First, I created a calendar that contains only the first day of each month, and only dates that current or in the past:

Program Active Months = FILTER(CALENDAR(DATE(2018,1,1),DATE(2025,12,31)),AND(DAY([Date])=1, TODAY()>=[Date]))

Then, I used this to produce the table I wanted which showed me enrollee count by client and month:

Active Enrollees = 
VAR tmpTable =  
SELECTCOLUMNS(
    FILTER(
        GENERATE(
            Enrollment,
            'Program Active Months'
        ),
        [Date] >= [Effective_Date__c] &&
        [Date] <= Enrollment[End Date]
    ),
    "Id", Enrollment[Id],
    "Date", [Date],
    "Account", [Account Name]
)
RETURN GROUPBY(tmpTable,[Date],[Account],"Count",COUNTX(CURRENTGROUP(),[Id]))

I hope this helps anyone else in a similar situation.