Weird extra member in facebook group chat?

[Annie]

I have a facebook group chat with me and 4 friends in it, we've had it for years and it must have over 300,000 messages in it. When i see the messages on my phone and look at the members section of the chat i see my 4 friends names and then this other extra weird name which I can not click on. The name/email address is: mailer-daemon@mx.facebook.com
It is not a link like my friends names and can not be clicked on.
When I log in on my phone I do not see this, it is just my friends in the chat.
What is it? because I'm scared.
This email address is appearing as one of the members in the group chat along with my friends, I have not received any actual messages from it.
There used to be more members in the group chat that left months ago, none of whom have blocked me. This extra 'member' has only appeared recently.