In "The Gathering" Lita starts in the Parlor and there are specific instructions on how the players can add her as an ally in the scenario. In "Curse of the Rougarou" and "Carnivale of Horrors" there aren't any instructions for "picking up" the allies during the scenario (I'm not talking about gaining them later as a campaign reward). Do these allies stay in their starting location or can the players effectively recruit them to run along with them within the scenario?

My guess is no because there aren't rules that allow the players to do so (at least that I've seen). Thanks!

In "The Gathering" Lita starts in the Parlor and there are specific instructions on how the players can add her as an ally in the scenario. In "Curse of the Rougarou" and "Carnivale of Horrors" there aren't any instructions for "picking up" the allies during the scenario (I'm not talking about gaining them later as a campaign reward). Do these allies stay in their starting location or can the players effectively recruit them to run along with them within the scenario?

My guess is no because there aren't rules that allow the players to do so (at least that I've seen). Thanks!

You are correct. They stay where they are. However, note that any Investigator at their location, or locations otherwise specified by their game text, may trigger their abilities, so they're not totally useless! As you've stated, you only gain them to use as regular Allies as part of the scenario resolution.