Migrate users from one site to another At the moment I've got 2 drupal 7 site. Now I want to merge those two sites together. If I export the user table in the database from one site and import the data directly into another site, will that be working? Btw, The users don't have any related contents like nodes or comments in the site so I guess the only table I need to worry about the user table. The only concern is the password. I'm not sure if I import the data the user password will be the same.
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."