By default, CometService is active for chatrooms as well.

You can manually activate COMET_CHATROOMS (set it to 1) if you have subscribed to CometService or any third-party transport in cometchat/config.php file.

When using CometService with chatrooms, credits are deducted for every message received and NOT sent. So if there are 20 users in a chatroom and one message is sent, then 20 credits will be used.