In Google Apps I can choose to Automatically collect respondent's username.
How do I display the username or First Name and/or Last Name on the subsequent form?
I'd like the form to read something like: "Hi Bob, thanks for taking our survey."
You cannot add any elements that are not available in the form builder, so you cannot control how the respondent's username is displayed.
But the the username is displayed to the user filling in your form, if you're using Google Apps for Business. Make sure the options Require [domain name] login to view this form and Automatically collect respondent's [domain name] username are checked:
The end user will see this on the top of the form: