Moodle has an inbuilt messaging tool for communicating with users of the platform. Note that this is an internal message (not an email) and notifications will be displayed on the dashboard page when users login to the platform.
Go to: Administration > Site Administration > Users > Accounts > Bulk user actions
Then find and select your users in the left hand side window, move them over to the right hand side window by clicking the "Add to selection" button and then from the "With selected users..." dropdown box list select "Send a message"
Enter your message into the Message body window and press "Save changes" once ready to send.
You will then be taken to a confirmation screen showing you the message and underneath it the names of all the users you have selected to receive it.
Press "Yes" and it will be emailed to the address associated with each user in Moodle