All they have on the site that I've seen is the following in the help section:
4. I lost my username for the GRFP application. How can I retrieve it?
You can click the Forgot Username link and retrieve your username after you have completed the screen form. Or contact the FastLane Help Central at 1-800-673-6188 (office hours: Monday-Friday, 7 am - 9 pm EST) or email fastlane@nsf.gov.
For some reason they deleted the Forgot Username link, but I changed the URL from the password one to give the following (which works):
https://www.fastlane.nsf.gov/grfp/GRFPForgotUsername.do?method=loadForgotUsernamePage&type=fellow