I use Qlikview, not Qliksense, but I have achieved exactly the same thing by:
1. Creating two tables, one with all the user activity and other with just the first activity per user (like Load user_id, min(date) from [activity table] group by user_id).
2. Then creating two calendars, one for each of the tables, that generate a separate Yearmonth variable for all the activity and the first activity.
3. Create a pivot table with the two yearmonth variables, and count(distinct(username)) as expression.
4. Drag one of the dimensions to columns according to your desired output.
Hope this helps!