MIM - A quick chat between more than 2 users
Room - An ongoing chat between several users
Nobody can add users to an existing MIM - Symphony is designed this way for compliance reasons. As a MIM is intended for a quick chat between more than 2 users, being able to add users would take away from the functionality of a Room. If you believe at any point that another user may need to be added, it is always best to create a specific Room.
Note: An IM will generate one conversation ID, however with a MIM when a user is added, it is considered as a new MIM and will generate another conversation ID from when the new user was introduced to the MIM