You can use the Quickmail feature in Moodle to email all or a selected group of your students:
In the middle of the page will be a box containing "Potential recipients." Those will be all the students enrolled in the course, along with any teaching assistants. There is also a Potential groups box just above that containing the name of groups in the course (if you have already set up groups). To send an email message to a particular group of students, you would click on one of the groups in the “Potential groups” box, and then you would click on the Add button to add all those students to the “Selected Recipients” list.
To send an email message to all students in the course, click on the button in the middle of the page: