This is a question often asked in the forums, so I thought it would be a good idea to show a quick walk through. There are two ways to contact all of your users at once when logged in as site admin: Notices or Private Messages. Let’s start with Notices, since they are the easiest to send.
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How to Send a Site-wide Notice to All Members
Under BuddyPress Component setup in the dashboard, you’ll see the description for Private Messaging “Let users send private messages to one another. Site admins can also send site-wide notices.” Notices are built into BuddyPress and accessible only to the the admin user.
If you want to send a site-wide notice, go to Messages >> Compose on the front-end. Compose your notice, and remember that it will need to fit in the sidebar of your site (or wherever your child theme is designed to display the site-wide notices). Make sure to check the box that says “This is a notice to all users.” Write and send your message, and then you’ll see that it will appear in the sidebar for all users. They will be able to click “Close” on the notice to hide it once it appears.
A record of your notices will appear on the front end under Messages >> Notices where you’ll be able to deactivate notices. Once deactivated, the notice will no longer appear in the sidebar when users log in to your site. The notices are saved there, so that you can activate them again if necessary. However, BuddyPress will only allow you to have one notice activated at a time.
How to Send a Site-wide Private Message to All Members
This method requires a little more effort, since it’s not built directly into BuddyPress. You’ll need to install a plugin called BuddyPress Mass Messaging. I found this little gem in a buried thread in the BuddyPress forums. It’s not available in the WordPress repository. The only change I’ve made was to the post URL, as it didn’t work for me. Download and install this plugin and you will find a new dashboard menu item under Settings >> Mass Messaging. This will allow you to send a Private Message to all of your users at one time.
The advantage of sending a Private Message over posting a notice is that the user will get an email notifying them about the message and it will cause them to be more likely to login to your site, resulting in higher traffic. Notices are a great way to make users aware of new features or items of less urgency. You can use both methods in combination as needed.