Well, prior to the briefing, only Holloway, Shaw, Vickers, and David knew what the ship's real mission was (and of course only the latter two knew the real real mission). Everyone else had been hired under some other pretense, apparently - though even they didn't seem to know exactly why they had been hired, based on the bet about a terraforming survey. I'd assume that the people at Weyland-Utani involved in the hiring process were also under the illusion that the mission was something other than what it really was.
As for the inclusion of a geologist, perhaps Shaw suggested that one be on the crew as an "in case" precaution - she, after all, took the presumed death of all Engineers much better than Holloway did. Maybe she anticipated that possibility, even though Holloway doesn't seem to have considered it. Or maybe it was Vickers or another Company executive who decided to include a geologist, on the chance that something of value might be found on the planet.
Even though I would expect Holloway and Shaw to have been intimately involved in the selection of the scientists, it's possible that Vickers somehow sabotaged the selection process to spend as little money as possible, considering she never believed in the mission. From some of her dialogue, it sounds like she may have expected to be running the Company upon her return to Earth.
As for the inclusion of a geologist, perhaps Shaw suggested that one be on the crew as an "in case" precaution - she, after all, took the presumed death of all Engineers much better than Holloway did. Maybe she anticipated that possibility, even though Holloway doesn't seem to have considered it. Or maybe it was Vickers or another Company executive who decided to include a geologist, on the chance that something of value might be found on the planet.
Even though I would expect Holloway and Shaw to have been intimately involved in the selection of the scientists, it's possible that Vickers somehow sabotaged the selection process to spend as little money as possible, considering she never believed in the mission. From some of her dialogue, it sounds like she may have expected to be running the Company upon her return to Earth.