You'll see there are 4 options when you create or edit a project.
1. Community projects
Free and open for anyone to join. Users who click the "Join Project" button will immediately gain access. Task doesn't require any subscription to run these open projects.
2. Team members apply
Your project will be publicly viewable to anyone viewing the URL of your project page. However in order to join the project team a user will need to click "Apply to join".
The Project Owner will receive a notification that the user wishes to join and can accept or reject the application, with the user notified automatically of the outcome. Requires an active Task subscription.
3. Team members are invited
Your project will be publicly viewable to anyone viewing the URL of your project page. However users cannot join the project unless they are specifically invited by the Project Owner. Requires an active Task subscription.
4. Private project, invite only
Your project will be NOT be publicly viewable - only team members of the project can view it, and they are on a strict invite only basis.
Typically you will want this setting for any internal business usage where you do not want to share data publicly. Requires an active Task subscription.