I have never had a problem with the intial trial flight with friends/family in the back.
I wouldn't advise taking them on any of the rest of the syllabus though. It would be quite boring for them.
I did once take someones grandad on some nav exercises but he used to be a tail gunner in lancasters and had done his training in the local area. He was happy looking at the hills and it was good experence for the student to have a few circuits at close to max weight in a C172.