A corporate pilot reported flying a VFR observation flight at 8;000 FT out of GJT in marginal weather and after receiving an EGPWS terrain warning; climbed through the overcast to VMC in order to obtain an IFR clearance back to GJT.

Narrative

Our mission consisted of flying empty to GJT to pick up three passengers. From there; with help from the passengers; we would navigate visually and observe the snow pack in the local area. Before the repositioning flight to GJT I had some reservations regarding the current and forecast weather at GJT. As I recall the GJT forecast weather was 6;000 FT to 8;000 FT BKN with showers in the vicinity; and a temporary chance of thunderstorms. Current ceilings were approximately 3;000 or 4000 FT. I referenced our operations manual and reviewed the section which states; VFR flight in turbojet aircraft not recommended. It also states that VFR flights are not permitted in marginal VFR conditions. I brought this to the attention of the Chief Pilot; who replied that GJT forecasts are well above the definition of marginal VFR. I asked if this flight could be completed on a day with better weather; later in the week; perhaps. The response was [that] we were assigned the trip and that is all that is known. The pilot in command did try to call the lead passenger before departing to discuss the inclement weather; but the passenger didn't answer. Upon arriving at GJT we executed the LDA approach to Runway 29 with low ceilings. I agreed with the pilot in command that a VFR flight under current conditions would not be possible. We waited some time and after lunch the passengers requested we make a go or no go decision. Current conditions were as forecast and had improved somewhat from when we arrived. The pilot in command suggested giving it a try and although I had some reservations; I didn't step up and use stop work authority as I should have. We followed the valley from GJT initially at 8;500 FT. After a short time in the valley we were instructed by the passengers to follow a canyon off our left side with higher terrain and lower visibility. I believe at this point we had climbed somewhere between ten and eleven thousand feet. As we followed this canyon; the conditions began to deteriorate. I noticed we were headed to the right side of the canyon and the terrain was rising to our current altitude. I suggested a turn to the left shortly before we received a 'Terrain; Terrain; Pull up' message. The pilot in command began a turn to the left while referencing the terrain and regained the center of the canyon. A few moments later; conditions further deteriorated and we lost all visual reference and inadvertently entered IMC. Altitude was noted at 11;500 FT. The pilot in command made a climbing left turn until VFR on top; approximately 2;000 FT higher. We had been with Denver Center for flight following the entire time and after regaining VFR; requested an IFR clearance for the return to GJT. The passengers never got to see the areas they had intended to view. However; they did say they received some useful information from observing the snow cover in the general area. The rest of the flight was completed without incident. I don't think the passengers we aware of the gravity of the situation. The ironic thing is that we received a pat on the back for going above the call of duty in accommodations for the weather. We received an award for poor decision making. I feel we put ourselves in a dangerous position and as an acting crew member I take full responsibility. As Safety Officer; I feel I should lead by example and in this case I clearly fell short. I had a bad feeling about the flight and stop work authority should have been exercised. I tried to express my concern by asking leading questions about the necessity and urgency of the flight considering the less than ideal weather conditions. I should have been more assertive. At any point I could have simply told the pilot in command I was uncomfortable with the flight; end of story. Looking back on the incident; that is where I failed. The major lesson learned for myself with this incident is this: if it doesn't feel right; don't be afraid to say so; regardless of seniority.