To add a specific user to all events in a recurring series in a Microsoft calendar:

1. Navigate to the calendar in the Microsoft application.
2. Select the series of recurring events.
3. Select Edit Series.
4. Search for the user you would like to add.
5. Select the user, and ensure that the toggle is set to On.
6. Select Save.
7. The user will now be added to all events in the recurring series.