1) Click the 'Post Notice' button from the Notices block in the main class page.
2) Click the 'Choose' button to choose an audience for your post.
3) Click on the name of your class in the separate section below the list of classes to navigate to this class.
4) Click on thebuttons to navigate to the group you want to target.
5) Use the tick-boxes to select your targeted group
6) Click save
7) When you return to your notice you can see the class:group is selected and you can then fill in the subject and content of your notice and send as usual.