From this page:
https://support.apple.com/guide/security/how-imessage-sends-and-receives-messages-sec70e68c949/
This is the server I believe they made them move to China to be under full Chinese jurisdiction. They can simply add their own key either to a targeted user or possibly to all devices. This is what I meant by direct key distribution. If you receive a private key in person directly from the device you’re talking to, you can be more sure only the two of you have the keys. That’s why Apple is introducing iMessage key verification, to be certain the key you have for the person you’re talking to is actually the key they are using. Doesn’t solve the distribution problem. The only way group iMessage can work is for Apple to handle the key distribution for chat members. That loophole allows them to silently add another “member” to the “group” chat.
That loophole isn't there for two reasons. First, each message is signed on the users device (second paragraph) so you know it was sent from the users device. This creates ID for the user to the receiver(s) devices. Apple
does control IDs for devices which I'll get to in a moment. You can't spoof a user sending a message because of this.
Second, each message is encrypted using keys for each receiving device. This means it can
only land on users' devices who are meant to receive the message including in group chats (the keys used to encrypt the messages can be for one or multiple users' devices.) This is described in paragraph 3. The diagram also shows how they are transmitted and do not hit Apple's servers.
The one potential "loophole" would be if an additional device were added to one of the group chat member's accounts. Apple
does control the ID servers. In theory when a new device is added to your account you'll get a notification on all your attached devices (we've all activated a new device to iCloud. That message.) Could Apple suppress that message in China, yes they could.
As this is written there's no way to get added to a chat silently to spy. I'll be happy to point out, though, that Apple's software isn't open so we can't verify for sure they haven't hacked things up differently in China. So truly anything could be possible since their servers exist in China for China.