By default, each hub has a feed where collaborators and participants can post messages visible to everyone in the hub. Find the one for this hub by clicking on the speech bubbles icon in the explore bar.
You can easily add rich content such as videos, events, livestreams and cards to your hub by attaching it to one of your posts in the discussion feed. If you haven't yet set any of these things up, you can even create them right from the post view. Any new content you add will automatically become available for use elsewhere in your hub.
In addition to the All posts feed, you can enable the discussion on any individual page. Use it to:
Create sponsor pages with a space for questions
Add discussion threads on specific topics
Allow comments on specific pages, such as blog posts
When enabling the discussion feed on an individual page, you can choose who can post and reply. Set it to Collaborators only to create an Announcements feed that only organizers can post to.
Collaborators have the option of sending a post as an Announcement, which means all participants will receive an in-hub notification and an email to their inbox and the post will be displayed on the hub homepage (unless you replace it with a custom one). To turn a regular post into an announcement, click on the Bullhorn (loudspeaker) icon before sharing.