You have multiple options you can use to communicate with a particular Group depending on your account's Admin status as well as if you are part of a group or not.
Posting to a Group - if you are in the group
If you are a member of a group you can put simply post to the wall of the Group in question, everyone will be informed that the post has been created. They can comment on the post, which is a great way to begin a discussion. All admin members are automatically placed by the system into the Ambassadors and Reps Group, so admins can communicate with each other really easily.
Select the group you want to share with by scrolling (on a mobile - see screenshot below) or using the drop down list (on desktop) next to the 'Post to" prompt to find the class or group you need.
Sending an Announcement - if you are not in the group
If you have been assigned an admin status by your Ambassador you can switch sides to the green Admin area where you can send an Announcement to a Group, Class, Year or even the Whole School.
This can be viewed by recipents as both an email and from within Classlist. Announcements allow a longer message of up to 10,000 characters, and can also contain an attachment as well. Announcements will be sent to all those who have been invited by email to join Classlist by an Ambassador or Class Admin, regardless of whether or not they have fully registered.