A very insecure method to be actually implemented in an applicaton (or in any application).
But, yes, if you want to login the users automatically. Then you can create a URL that redirects the user to such a page where the UserName and the Password would be automatically set to login the user.
Tip: Instead of using a username/password there, you can use a token generated by the user each time he sends the email to others.
The sh*t I complain about
It's like there ain't a cloud in the sky and it's raining out - Eminem
~! Firewall !~
When you write your insert statement, don't attempt to insert a value into the fields that have identities set. Identities are automatically created for you by the database server. If you want to get the value that you created back, you simply use
"When I was 5 years old, my mother always told me that happiness was the key to life. When I went to school, they asked me what I wanted to be when I grew up. I wrote down 'happy'. They told me I didn't understand the assignment, and I told them they didn't understand life." - John Lennon