By default, all organization members can access all organization projects. If you're working with 3rd party developers or designers though, you can still add them to the Organization as Restricted members. Restricted organization members can only access the projects you invite them to.
After adding Restricted members to the organization, make sure to invite them to the projects they need to access separately (from the project dashboard).
Restricted members also count against your total number of seats.
If you're adding new members from the organization dashboard, you can select “Restricted” while inviting them.
If you're adding Aliens as Restricted members, make sure to click the dropdown next to the “Add to Organization” button and select “Restricted”.
You can also change a user's access level after adding them to the organization. When you hover over a user from the organization dashboard, you can click the icon (👁 or 🔒) to do so.