I have a question about movement in the game. I know that once you resolve an adventure, your investigator returns to the entrance. What if you fail to resolve? Does your investigator stay on that card until his next turn? I’m trying to figure out how more than 1 investigator can be on the same adventure. Sorry, newbie here and I’ve gone through the rules and don’t see anything specific.

Your investigator stays on a failed adventure. You can opt to move away from it during the movement phase of your next turn (except for a couple expansion adventures where you're forced to stay until you succeed or die, or the AO awakens). I took a look at the rulebook, and I think you're right- this isn't mentioned specifically. However, the implication is there, since the movement phase occurs before you try to resolve an adventure, and the rulebook only tells you to return to the entrance if you succeed.