Deepali I’m not sure your question makes much sense at least to me; the “Created By” related attribute on your “Closure” business object IS a reference to the user object itself and represents which user pressed the “Close” action in your process.
Are you wanting the creation date of a particular user profile instance then?
Like Julian said, it's simply a relational attribute from User to another object with a persistence type of "Creation User". If you drag the user object over to any other object, drop it, select No to the question it'll pop up a Persistence Type box. Move Creation User from left to right and say OK. This tells SQL to capture the user that created a particular record (like resolution, closure,note, attachment, etc..)