Give it a few seconds but the players' usernames should sync up. If they don't then reload the page.

This splits a username into 4 segments of 4 characters each, then saves those as numbers using the Shared behavior block. This then utilizes the fact that all the Shared blocks will automatically output their value when a new user joins and syncs up, so they will share all the necessary username information with all instances of the game.

This is the username sharing system used in Misty Hollow (https://flowlab.io/game/play/2388181)