You would include them at the end of the survey link…
Original link:
http://www.surveygizmo.com/s/12345/my-survey
New link:
http://www.surveygizmo.com/s/12345/my-survey?username=deltina
This is captured in the survey as a URL TOKEN, and is included in the .csv or .xls exports. If you want it as part of the summary report, you will want to add an ACTION > HIDDEN VALUE to your survey and set it with the merge code for URL tokens, which using my example above would be:
[%%URL_username]
We hope to compile a more specific tutorial in the near future, so keep an eye on http://www.surveygizmo.com/tutorials/