I think there are a few plugins that do that. Here’s one that I found in a quick Google search:
https://wordpress.org/extend/plugins/email-users/
I guess I should have worded the above different. I also found the plugin you speak of, however–I guess what I was wondering is, will buddypress eventually incorporate their own plugin. The reason I state that is because, it seems logical to email all users in case there is a problem with logins or if someone hacked the system. BuddyPress seems like it should be the responsible party to potential problems. Not muWordpress or wordpress. And being that it is a social network website owners must keep the attention of users by sending information out occassionally. Just a thought…
Well you could create a site notice. Just go to your profile’s message area and click “Compose Message” and check the “This is a notice to all users.” checkbox. I also bubble these notices to the homepage by adding a simple template tag to my home.php template:
<?php bp_message_get_notices(); ?>