I think this is one of those small things that should be added to BuddyPress core, but the following snippet allows you to change the default Notification settings for new users. This snippet is taken directly from the BuddyDev blog. Buddydev is run by my friend Brajesh and his team who’ve been providing the community with free and premium plugins for years.
To change your BuddyPress email notifications settings, add the following code to bp-custom.php
Note: This functionality is also available as a premium plugin on BuddyDev. Check it out!