Punchout Catalog - Need to pass userid information to catalog site.

Hi Guru's

I have a question.  Currently all users go into the Staples punchout catalog.  We have a requirement to be able to pass the userid or email address of the users to the Staples catalog when they go into it.  How can we pass say the email address of the user to Staples site that is shopping from SRM via the punchout catalog?

Thanks,

Jeff