How to identify google forms user?

Question

Hi,

is there a way to know, who created an entry in a google spreadsheet via a form without giving them an account to our google apps domain?

I am working a system to distribute users to different groups.

1. first the users have to input there preferences
2. the users are distributed between the groups with regard to some contraints like group size
3. the users have a possibility to change there preferences
4. regrouping

I have 'everything' worked out in google docs, but I cannot ensure, that some users re-enter the preferences of other users maliciously to take up their place in the groups. Could I identify the user who made the entry, I could delete unauthorized entries.

What do you recommend?

p.s. I hope, I got the tags right.