Assuming you are thinking of SEP on the North Atlantic, if a ferry tank is installed then it is 'required crew only'. No pax thus. It does beg the question slightly of what constitutes 'required crew' but any ferry company wanting to remain on the correct side of the law and in business would say P1 only. Cockpit is usually very cramped with ferry tank anyway.
With standard configuration a passenger(s) is possible but not encouraged. I have let the owner accompany me once transatlantic and it is the last time I do it. Confusion of role and status (his) was very wearing and potentially dangerous at times.
Pick your aircraft carefully too. 1000 nm range recommended. Quite apart from reliability issues, operating inevitably through Greenland is not for the faint-hearted. I cannot recommend Narsarsuaq as a first time routeing as the nearest alternate (Nuuk) is 250 nm further; this is on top of about 680 nm from Goose Bay. Extra weight reduces range. So you have to go further north via Iqaluit-Sondrestrom (490 nm) or Kuujjuaq-Nuuk (610 nm). No fun in winter. Transit of the ice-cap can have MEA of FL150 dependent on conditions although if you are lucky enough to get guaranteed VMC you can scrape by with quite a bit less - maybe 11000ft. So forget C152s !
You could try pestering the ferry companies (please do not mention me) but your best bet is to bring forward your purchase and make the award of the ferry job conditional upon your tagging along !
Finally, for a first time, do it in summer, July or August. Less icing, warmer water (!) and more daylight for the photos.
It is awe-inspiring (and a drug).
SI