I know it is not legal to ride in a normal trailer when being pulled, but can you have passengers in the 5th wheel trailer while driving?

Absolutely no way, no how, no time.

Sometimes there are trailers built like a bus for carrying passengers, but they are special rigs and are licensed specially. Unless you have something like that, the 5th wheel attachment won't make any diff legally.

You can only ride in a seat with a seatbelt legally

State laws do not permit it. I doubt there is one that makes exceptions, it is simply too dangerous.  

Depends where you live and what the safety precautions. It is illegal where I am 

Laws vary worldwide.

Many countries have no specific laws against that, but insurers would make clear that all passengers must be in a vehicle designed to carry passengers. 

Countries which do have laws about that usually ban that practise with very few specific exceptions, mostly relating to agricultural vehicles using public roads for short trips between fields.

The vast majority of trailers are not designed to carry people and that includes fifth wheelers: they don’t usually have seat belts and the necessary hard fixing points for those, and they’re not crash-tested for passenger safety.