Continuing the discussion from The Society's Community "How To" Guides:
Sending a message to a group is very similar to sending a message to an individual user.
To send the message, go to your ‘messages’ page by clicking on your avatar in the upper navigation (top right) and select the envelope icon for ‘messages’, or view your profile and select ‘messages’ from the navigation.
In the first box, include a group name by starting with the “@” symbol then typing or selecting the group name, then write your message and give it a title as normal.
Each group can have different rules about whether you need to be a member in order to send a group message, or whether group messages are even allowed, so please check the group settings.
These group messages are sent to all members of that group, and will appear in members’ inboxes in a separate folder for that particular group.