@jd131111 This one is when you add a clone to the game with the add sprite to game as shape block, it will run the “when added to game” hat block as that clone which is then spawned on all clients, also it makes it so whenever a new player connects to the game, it will spawn that clone for them and run the “when added to game” on that client example below
(In this example assume that you set in the prepare for green flag hat block to create/join the same world, so everybody is connected to the same world)

On green flag click /
Create clone of myself with id (username)

When I start as clone /
Add this sprite to game as [rectangle]

When added to game /
log to console (clone id)

when the first player (lets say their username is “playerone”) joins, it will log on their client “playerone”
When the second player joins (lets call them “playertwo”) it will log in their console (playertwo’s console) the message “playerone” because the clone that the first player added to the game gets spawned on this players client and runs the when added to game block,the then both clients (playerone and playertwo) will log “playertwo” because playertwo’s clone is now added to the game (edit: and spawned on the other players client), I can make a demo project if this is too confusing (edit: you repeat this logic for all players, and a side note, the clones positions are synced when they spawn in, but to keep their positions in sync you have to use the method shown in the 2 player space shooter example by CreatiCode, where you rely on the sync set speed to move them)