Organizations can identify default public forums that any new members will be automatically added to instead of manually adding each new person to forums that are relevant to everyone in an organization.
To add default forums that new members will be added to:
Go to your organization on the left-side navigation
Select organization Settings
In General settings, go to Default public forums and choose which forums you’d like to to add new organization members to automatically
You can choose to either add new members to the 15 most active public forums or manually choose a default set of public forums.