Congrats on getting almost to the end!
Join the shoulder seams across all the stitches. Don't leave any sts unworked here (although it's an interesting idea). The next part sounds like the beginning of the collar or neckband. Rib the sts from the right st holder, pick up 2sts over the shoulder seam (one before the seam line and one after it), rib the back sts, pick up 2sts over the left shoulder seam and rib the left front. I think the sts are picked up to prevent a gap at the seams on either side and maybe to help the stitch count for the ribbing. Is there a pattern link or name for the sweater so that we can take a look at it?