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