The BuddyBoss Platform and Theme has the Group Messaging feature. This will allow users and admins to send bulk messages. You can check this tutorial to learn more:
https://www.buddyboss.com/resources/docs/components/groups/group-messages/

You can also try using the Site Notice feature that allows you to instantly send a notice to all users at once but, this will not be received in the member's private message inbox but a banner-like display on your site. 
You can check more details here: