running totals count

Try the Running Total quick measure. You could potentially replace the SUM with COUNT for the aggregator. If you can post sample data could provide a more complete answer.

I belive you are looking for a measure like this:

Running total of unique users per month = CALCULATE(DISTINCTCOUNT(TableName[UserColumn]), FILTER(ALL(TableName), TableName[Month] <= TableName[Month])))

More information on the data you are using would be helpful.

Hope this helps!

Distinct users = CALCULATE(COUNT(Table_Name[User ID]), FILTER(ALL(Table_Name), Table_Name[Month] <= MAX(Table_Name[Month])), VALUES(Table_Name[Category]))

This measure will display a table like this:

To 'fill-in the gaps' as in your example I think you would have to use a calculated column which is more difficult (and I am unsure of). This will give you the same results but leave the cells blank if no change has occured. You may be able to find a way to fuill in these gaps.

Hope this helps!