Observer is a helpful user account type for users doing cross-project collaboration. With this type you're now able to configure globally if users can view all projects which they aren't a part of.
The Observer user type has view-only access to all public Projects and Teams in the systems (as opposed to the Contributor user type, who has edit-access access to all public Projects and Teams, and regular Users, who have access only to Projects and Teams they are members of).
Observer setting immediately provides a user with view-only access to all public Projects in the system. To provide a limited access to selected Projects only, we recommend to introduce Observer Role instead.
Make a User an Observer
Administrators can make others Observers one by one. For this purpose they edit user type from the Account Settings tab of a user's details view.
Access to Data
If an Observer user is not a project member, then the view-only permissions are applied. If he is a project member, then her Project Role permissions are applied.
Access to Views
Observer setting does not influence access to Views. When a View is shared with Project or Team members only, then it is hidden for Contributors who are not members of the Project or the Team.