I am no expert and I am sure someone will come and help you soon but this is what I do. Have you seen how to join DPN? There are different ways but I think the one where you transpose the first and last stitch. I use a crochet hook and take the first stitch and put it on the third needle and take the last stitch and put it on the first needle. In other words where you join just cross those two stitches. Now, for magic loop you are just suppose to knit the first one tight but I still found a gap. So I did the same thing as the DPN. After you divide the sts in two I pushed the sts close to the tips on the needles and took a crochet hook and transposed the first and last stitch. Then I pulled the right needle out and knit my first 3 stitches tight with the tail. You can not even tell where it joins.
I am sure someone else will have a better way but that is just how I do it.