Roles and permissions in phabricator.wikimedia.org.
Anonymous users can view all the public information as read-only.
Registered users can perform all the common activities: create and edit tasks, comment, upload mockups and files, edit their own profile...
Any Wikimedia SUL and any Wikimedia LDAP user can register.
Administrators can do and break a lot, although in Phabricator they are not all-powerful by design. They can access protected data (except your password), and they can make it accessible to others accidentally. However, administrators can not bypass object access policies.
For these reasons, membership of the Administrators team is very restricted. No one can be an administrator in Phabricator without signing an NDA. A process to join/leave this team must be defined. Also see the (now obsolete) Bugzilla administrator rights policy.
See the Help page to learn more about using Phabricator.