You can make a team private or public from the team settings page:
By default, all teams are public to the organization — any organization members can view standups or check-ins that are published to that team. With public teams, any organization member can join or subscribe to a team.
When set to private, only team members will be able to see the team or view standups or check-ins published to the team. An existing team member will need to add or invite new members.