Yes it is! It can also carry an implied sense that you have experienced what they are talking about. for example. Myself and a work colleague both arrive late at work. They say to me. You're late too huh, the traffic was a nightmare on the main road. I reply I know, tell me about it. Another example; Talking to friend A about friend B. Friend A says to me "She has been so moody lately." I reply, "I know, tell me about it!"

Tell me about it- Is more of a reassuring phrase that shows you are concerned and want to hear what the other person is going to say when they hint they want to share something but are not sure if you want to hear it. The phrase is more of a response phrase. You don't really start a conversation with it. Hope this help. Please wait for more answers. I am sure someone give you a more grammar oriented answer.