Ah! I had the same problem, I am a member of "many" groups of which friends added me there. So problem is I can't leave, because I don't want to dis-heart them.
So I had one idea on how to know without checking all the messages and going to Chat Rooms etc. Just Click on
Group Icon, Select
Info Icon now and there you will find
Media. Yeah with media now you have
Name of sender. You can actually check all the media without knowing them that you read the messages. :p
I don't think it's exact answer of your question, but it's called 'Work Around' LoL