Hello, Rae Ann!
What might be confusing is that it reads "The remaining front stitches are then cast on to complete the circle." Actually, two things happen on the same row (cast on AND join the circle), but this sentence makes it sounds like the casting on does the joining. It doesn't.
As you knit back and forth, adding matching increases at the two front pieces, the front pieces of the sweater keeps growing toward each other. If you keep on doing this until they meet, the result is a V-neck; when you have all the stitches that you need, then you simply join and switch to knitting in the round.
Or, if you want a rounded neckline, at some point you cast on two or three inches of stitches that will be in the middle of the front and then join in the round. I am making a t-shirt like this right now.
Best wishes to you -- it's a fun way to make a sweater.