Setting the visibility of a team to public or private
Team admins can make the team's visibility public or private. You can change the setting in the team's profile page. Click the "Settings" tab and then select "Team Visibility" to make the changes.
Private teams will not show up on your profile when other people view it. They will also not show up in search results.
Public teams have a public team profile where visitors can see the team's members and a list of their public boards. For example, see https://trello.com/trelloinc.
NOTE: Although the team is considered 'public,' only those boards within the team that are also 'public' will be visible to external users.