Program Chairs can message any venue participants through the group consoles. Clicking any of the links under 'Venue roles' on your PC console will bring you to a console for that group. If you click 'Edit group', you will see the option to email those group members. You can customize the emails using the backend tags. Note that you will not be able to use the author group to message authors until after the submission deadline.
You can use the PC console Paper Status, Reviewer Status, or Area Chair Status tabs to message selected reviewers or area chairs, respectively. You will also have the option to message only those with incomplete reviews/metareviews, only those with completed reviews/metareviews, or only those assigned to particular papers. You can customize the emails using either the frontend or backend tags.