You have a few options.
With most groups, if you don't give any explanation and just tell everyone else, "yes, things have always been this way", then they probably won't immediately figure out what's going on or why. There's a few theories they might come up with, and they also need to figure out what the trigger is for a reality jump. It's one of those things that's obvious to the GM but the players may even need a push to figure out. This is the option I like best.
You can pass around notecards. Since players won't know what's on each others' notes, they may suspect each other even when they seem to agree. I don't like this option as much, but it's doable.
You can always just get everyone's buy-in out of character. This will remove the mystery, which I don't like, but in practice a lot of groups will have just as much fun.
I'm glad you like the module, and I'd enjoy hearing what you plan to do!