I've sent a message in Facebook to multiple people. I want to send another one to the same group without adding them one by one, but just add one new person instead. Is there a way to do this?
This is not possible in Facebook. It's by design.
In the message at the top right there is an actions button with an 'add people' option.
I seem to have a newer version of the message function (as it keeps telling me most of my friends don't and I should invite them). So sorry if it doesn't work for you.
It is possible, click actions and then open in chat you can add people.