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/class
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.
Sending an Announcement - If you are not in the group/class
If you have been assigned 'Class Admin' status by your 'Ambassador' you can switch sides to the green Admin Portal 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' to join Classlist by an Ambassador, regardless of whether they have fully registered, making this a great option over a standard mailing list.