Hi Sirry,
we built a workaround to get the user. Don't know there will be an easier way.
You could build a "external input" parameter for user and on presentation you could set the parameter with: "Default value": "#__username" to be set as with the username of the person answering the user-interaction. You could also set the input as hidden, so the user don't know he is providing his username. "Hide parameter input": "true".
Regards,
Chris