There are private and public projects.
Only people assigned to the project can see it. The project Owner can manage who has access to the project in the Settings view.
Here is an example view, where the project Owner can manage project members:
Public projects are freely available to view by everyone who has access to the Internet.
Also, in case of individual workspaces, you can invite collaborators to public projects only. Examples are:
Click Project at the top-left of the window.
In the pane that appears, click New project.
Set a name, color, description and project type (Public or Private).
The new project is created.
In the Project panel, click Settings, then click the People tab.
The tab displays the list of project members who already belong to the project.
Above the list is a field in which you can type an email or username of people you want to assign to a project. Neptune identifies names and emails for people who already have an account.
Specify an email or username of a user you want to add.
Set a role and click Invite.