Managing private message participants

I’d like to find out if there is any mechanism to configure a private message topic to prevent the target recipients from inviting additional users or removing themselves from the private message topic.

I’ve been trying to create a system-managed private message topic where I use API calls to automatically invite/remove Discourse users based on their membership in an external group.

When a new external user joins the external group, their corresponding Discourse user gets invited to the managed private message topic. When the external user leaves the external group, they are removed.

However, I cannot prevent an invited user from inviting others through Discourse or removing themselves from the private message, so I cannot strictly manage private message participants.

Could I be missing something, or is it not possible to manage private message participants in this way?

Thanks for your help!