What is a Broadcast only chat group?

A Broadcast only chat group is a group where only Admins and Moderators can send messages; Members cannot post or respond in the group. Whether or not a group is broadcast-only can be toggled on or off at any time in the settings by the group's Admins. Broadcast only groups can be private or public.

Broadcast only groups differ from the Announcements groups in the Federation / Community News section of the Home tab in that those groups are view-only. Federation / community members can see messages sent in the relevant Announcements group (if it is public), but they are not actually "in" the group.

This is further evidenced by the fact that they cannot view the group's settings, whereas members of broadcast-only groups can.