I've done it before, but barely remember what I did. I believe I used some form of slip stitch or did it like a provisional cast on then knit into it.
I do the provisional cast on from here where you use a crochet hook. I think the KH one is different, but this would work for a ring.
http://www.knittingatnoon.com/demos.html
Also... what is the name of the pattern? Do you have a link? That always helps when asking questions.