Assuming you have already logged into phpBB. Click on the System link found on the main menu.

Then click on the Mass e-mail link found on the left side of your screen, under General Tasks.

Generally mass e-mails are great for announcements or notices, so that many people can be notified at the same time.
Ensure all users are selected in the send to group, so the e-mail reaches all users.
Enter the subject, the message you want to send and click the e-mail button at the bottom.

That’s it, you now know how to send mass e-mails to users in phpBB