You can collaborate with other users on a private project by making use of organizations. You achieve this by creating an organization, adding your collaborators as members, and then transferring the shared project to the organization.
Any operation performed on a project shared through an organization is applied in the organization's namespace, and is visible to all members. This includes uploading new revisions, specifying substitutes, supplying pricing, marking components as consigned and placing orders.
Viewing Your Organizations
On your dashboard you will see there is a widget that lists all the organizations to which you belong. Additionally your projects and orders lists will show any projects and orders associated with your organizations. Organization membership can be modified by clicking on the organization name and a new organization is created by clicking on the New organization button.
Managing Organization Membership
You can manage organization membership in the members list on the organization's profile page; reached by clicking on its name on your dashboard. You add a collaborator by entering his username in the input box next to the Add button. If the username is valid the input area will be green and if invalid it will be red. You need to know the usernames of your collaborators in order to add them. To remove a member click on the small x that appears next to the name when hovering over it.
If you belong to an organization and you are viewing one of your projects then there will be a "Transfer Ownership" section on the settings tab of the project page. The drop down button can be used to select a target organization to which ownership of the project should be transferred. Similarly when viewing one of your organization's projects you can transfer the project to any of the organization's members.