To send a message to a specific user: when you are in the Users & Roles settings, click on Users, search the name of the user you want to message, then click on the arrow next to their name. A list of options will appear underneath their name, and you can click Email to send them a message.

To send an email to all users of a certain role, navigate to the Users & Roles settings, click Notify, and you will be able to check off the boxes of the types of users you want to notify (i.e., readers, authors, reviewers, section editors, copy editors etc.). There's a box at the bottom of that screen to type your email message and add a subject.