Send a message to multiple users
With each message, you can select multiple users from the active users list. You can also send messages using the "Everyone" group or define your own custom groups. Right-click on an existing group to edit that group, or click on a grey "Create Group" to create a new Messaging group. You can only add Messaging group members when those users are online ("Active Users)".
Keep in mind that each user will need to create their own Messaging Groups at each SideNotesIM client. When you create a Messaging Group, this group only exists on your specific client.