This is how I see things, you have this page for users to register for the sweepstakes. To register they must already be registered in your sweeps_registration table. When they fill in the form you will have a recordset for the user that is filtered on the entered email address. It would be a good idea to also filter this on the password entered to ensure that it is that user who is signing up for the sweepstakes.
I think you could do this in the following order.
Have the form post to itself (done)
Have a recordset toward the top of the page that is filtered on the user's email address and password. These are the form fields for the username and password.
Next is the validation, you will need to validate the total rows from this recordset to ensure that there was a match. (done)
Finally after all of this you can have the insert to insert the user into the sweeps_entries_tracking table. For this insert you will be inserting the same user information that you already have in your sweeps_registration table. Since you have the rs that is filtering this table based on the email and password you can use the bindings from this rs directly in the insert.
That is all that you need to accomplish this, in the end you will have a form that a registered user can get to and enter their email address and password. The information will be posted back to this page and filter a recordset. It is this recordset that will be used for the validation to ensure that the user is registered. You will also use this same recordset in your insert server behavior.
You should start this by editing your existing rs to be filtered on at least the form field for the email, and the password as well if that is an option. Once you have got that part worked out the validations should already be working and you can move onto the insert. Please post back with any questions that you have and we can help you get things worked out.