Creator Customer Portal - Lookup Portal User ID by Email Address

Creator Customer Portal - Lookup Portal User ID by Email Address

I created a form which creates a Portal User via Email Address and assigns a user profile based on selected radio button (this part works great). I'm using the record of this as my own "User Profile". I've added a "Users" field but I can't get that field to auto populate. I assume it needs the ID of the portal user, but I am unsure of any deluge code which allows me to lookup the userid by email address.