You can only log passenger time on an aircraft under 5,700 kg for which the POH specifies a minimum crew of one pilot and on which you are not endorsed.
I disagree.
Some contracts specify two-crew ops required in a Chieftain, for example. If the second crew member is there performing flight crew duties, are you saying he cannot log the time?
If you are performing co-pilot duties, you can log co-pilot time.
That means, however:
1/. working for a company that has
2/. trained and appointed you
3/. to perform co-pilot duties
4/. on an aircraft on which you are endorsed (*)
5/. in accordance with the procedures described in your operations manual and accepted by CASA.
* ...and if there is no such thing as a Co-pilot endo, you must hold a command endo... and you must hold a current rating for the operations undertaken, ie: current CIR if the ops are IFR.
It might not pay well... it might look like a joke... employers may not take you seriously... but the holder of a flight crew licence
MUST log the time spent performing flight crew duties.
I might add that the scenario proposed by the original poster does not meet the requirements outlined above.
I know of a chap
with a couple of hundred hours co-pilot in ME T/props that can't log it because he wasn't endorsed
