The Union Platform (tm)

Given the correct privileges, a client should be able to send messages to rooms it's not in.

Reasons to allow outside messages:

• a moderator broadcasts a system message to many rooms without first joining them
• inter-application communication (e.g., a player in one chess game sends an invite to all players in another game)

Reasons to prevent outside messages:

• developers might want to turn off "outside messaging" in order to prevent clients from interacting with rooms they're not in.

Security

Outside messaging needs to be restricted by Union's general security/privileges model. A client should not be able to sabotage an app or spoof operations by messaging a room without being in it.