Private project is a project that is only visible to a limited set of users.
1. Make your project inaccessible for anonymous users. To do that, either ban the guest account or configure guest user roles so that guest users don't have permissions to view the issues in your project.
You can't ban the guest account with a legacy 10-user free plan.
2. Make your project visible to the desired group of users only:
1) Grant roles in the project to the desired users and groups: open the Edit Project page > Access tab > 'Grant role'.
2) Make sure that only project team members are added to the project team: on the Edit Project page, open the Team tab and check the list of members.
Membership in the team grants users access to the project based on team roles. By default, it's the Developer role.
3) Review the roles assigned to users on the Global project.
4) Revoke any roles granted to the 'All users' group in your project: open Administration > Groups > All users > Roles tab and revoke roles.
5) Revoke any roles granted to the auto-joined user groups in your project. The procedure is the same as in p.4. Auto-joined groups are marked with a corresponding label on the list of groups.