I will be the second Giraffe, saying that using included collections is almost always better than trying to write your own.
But I think your original question can be changed a little differently ... "Do I need a separate class to manage users other than the User class?
I use the static factory class to collect all my users and save them back to the database again. I believe that your model classes should be as deep as possible and that you use heavy controller classes to do all the work for the model classes.
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