Migrate users from one site to another
You need to use Drupal-to-Drupal data migration module. Project page says 

"With migrate_d2d 2.1, we are introducing a wizard-based UI for defining your Drupal-to-Drupal migrations. Enabling the migrate_d2d_ui module will add an "Import from Drupal" subtab to the Migrate dashboard - visit that tab, enter the credentials for your source Drupal database (versions 5, 6, or 7), and follow the steps to define how your legacy content maps to the destination Drupal 7 site."