I do also like the idea.
And if I understand it correctly, it looks as if the “filtering” UI could be the same for the contacts within the chat-list view, as for the subjects within the chat view (see there for a description). The difference would be that a new tab “contact groups”? in the chat-list’s title context would allow to add new groups and group members, while the “topics” tab in the chat title context would allow to add new subject topics.