Looks like they're using a string of sequins on thread and catching the thread from the bottom with a very small crochet type hook- which, from what I can tell but haven't tried, allows them to lay them down side by side. I'm guessing the crochet hook allows them to have make a type of chain stitch stitch on the back side- to lock them in place.

I thought that to at first, but then got to thinking....it has to be some kind of needle and hook or something like that. She comes from under and thro the fabric to catch another sequin bead.