WordPress allows you to publish password protected posts. You can add password protection in your blog post by using visibility option present in the Publish meta box on post’s add new/edit page; using this feature you can set a separate password on your post and share it with selected users. Now this tutorial explains you how you can set a global password which is applicable with all your password protected posts and pages.
Install and activate Global Post Password plugin. After activation visit its options page and set your global post password.
Now you all you need to do is select Password Protected option is visibility section of Publish metabox present in post’s add new edit page and you are done. Note: If you will change the global password then all your password protected posts will get automatically updated with the new password.