There are indications in the book that there is a common communications channel, as well as a linked pa/speaker type system. Add to that, there was very extensive media coverage, due to Parzival's 36 hour media blitz before the assault.
During the arrival to the battle at Castle Anorak, the relevant passage from the book is
A cloud of smaller ships quickly formed around me, piloted by curious avatars zooming in for a closer look at Leapordon. I had to mute my comlink because so many different people were trying to hail me, asking who the hell I was and where I'd picked up such a sweet ride.
Once they land, there are thousands of avatars already on the ground around the shield:
When I got closer, I spotted an open patch of ground directly in front of the castle's entrance, just outside the shield wall. Three giant figures stood side by side at the center of the clearing. The crowd around them was continuously surging inward and then receding as avatars pushed back against each other to try to keep respectful distance from Aech, Art3mis and Shoto, who each sat inside their own gleaming giant robot.
The most relevant, however, is when Sorrento (Head Sixer) addresses the group just before the shield drops
Sorrento surveyed the scene, them smiled up at us. When he spoke, his voice was amplified through powerful speakers mounted on the Sixer gunships and hover tanks, allowing him to be heard by everyone in the area. And since there were cameras and reporters from every major newsfeed outlet present, I knew his words were being broadcast to the entire world.