You can post common information and comments to all participating designers in the "Announcements and public discussion board" section on the bottom half of your contest page. This section is fixed at the bottom of all tabs i.e. Design Brief, Entries, Messages, Designers, Stats, Polls & FAQ. The comments posted in the "Announcements and public discussion board" section are visible to all the designers.
In case you want to message the designers individually, you can do so by commenting on their designs under the comments section or by clicking on their names under the Messages tab.