Lethe doesn't have that ability. Her voice isn't the one speaking to the characters in their dreams. Lethe is currently out finding the other people. She mentioned there would be more. I'll spoil it a little for you if it'll help ease your worries: The woman speaking to Nil is one of the others. She's also helping gather people at Lethe's request. 

I had hoped Magus and Frore would help with the search in some way, but since that isn't the case it's a bit of a scramble right now. Jira ditching Felica so quickly was something I hadn't anticipated as well-- it would have been far easier for Cain to direct them both to the graveyard while they were in the same room. I'm banking on every player's tendency to have their characters meet up and interact with other player characters. Thus finding people individually would be less common. (Obviously that's not the case with Nil and, to an extent, Maryev--but that's totally fine.) Naklov, Kaiver, and Mila are all bunched together however. As are Ekra and Angus for the time being.

And don't worry too much about what's going to happen once everyone is at the shadow temple. I have something in store.