Welcome to KH!
Thanks very much for the link. It's so helpful to see the finished project.
It's close to a 3-needle bind off as Trisha suggested, but without the bind off
. Once you have the two pieces, the neck edge sts on one needle and the ruffle sts on a second with RS facing each other, use a third needle to join the two. Insert the needle into the first stitch on needle one purlwise and then into the first stitch on needle two. purlwise and purl 2 together. Then repeat for the second sts on needles one and two and so on. By the end you will have joined the two pieces together. Follow the directions for the next row and then the bind off.
Here's a video that shows the set up (for purlwise, see about 3:42 min). Don't do the bind off step,
just use the video for the set up and purl sts. You should have stitches on the third needle after purling 2 together.
It's really a darling pattern. ,