Hello Amit,
Welcome to the Atlassian Community!
Out of the box, it would be extremely hacky to do so. You would essentially have to have a script to run an API call to add everyone as a watcher to a pull request as its created. Not elegant and would most likely make a lot of people angry.
Instead, I would recommend using ScriptRunner for Bitbucket Server. Out of the box, it has a PR merged event where you can send emails. You can then just add all of the users to To addresses.
To make it a bit easier to work with, maybe you can send it to a single email address that would then forward it to a group that has all of your developers in it.
Regards,
Aaron Levinson
Dev Tools Support Engineer
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