After you have begun a private meeting in a channel, you can make it public by generating a meeting link and sending it to users outside the channel.
To change a meeting from private to public:
Send the link to anyone you want to invite to join the meeting.
Once you click Create Link, the meeting becomes public for everyone in the meeting. Attendees who were in the meeting when it was private no longer have a connection between the channel and the meeting. When they click the Message icon, instead of returning to the channel, they open a message panel on the left.