Join really means just take the new yarn and start knitting with it, leaving a 3-4" tail to be worked in later. So knit the first shoulder, drop that yarn, pick up the new end and BO the center sts and continue across to the other shoulder. Turn and work the WS row with that same yarn until you get to the neck stitches, drop it and pick up the first yarn and work the other shoulder. Turn and repeat.
Hope that helps.
sue- knitting heretic