There are a few ways that this could be accomplished. The exact nature of this would depend on your programming language, but the concepts would remain the same:

1. Session variables/Cookies. A cookie/session variable housing the appropriate could be read from a user's computer and processed as needed. If cookies are disabled, this functionality would be disabled as well.
2. Querystring. Pass the user id from one form to the next via querystring. The problem with this method is that someone, either by accident or on purpose, could elect to alter the querystring and thus cause processing errors within the form.
3. Hidden form input. This is the one I personally prefer. A hidden form field would generally be more trouble to modify than it's worth, and in the case of a multi-part form would have to be isolated and modified multiple times. Mind you, this is only true if you're POSTing your form information. Do not GET your form information, or you will end up with a querystring that, as stated above, can be manipulated.

Hope this helps.

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