There is a JIRA Project say 'Project Demo'. I would like to add user1, user2, user3... as default watchers on all tickets under 'Project Demo'.
Is it possible. How should I enable it, if yes?
Go to your project page, select "settings", then select "Autowatch Rules"; this will give you the ability to set up JQL, e.g. "project in ('Project Demo')" and select the users you want to have as automatic watchers.