The videos on this site show several ways to join different colors. If this link doesn't work, go to Basic Techniques/More and scroll down.
I'm not aware of any technique that will allow you to add a new color without it showing on the back of the knitting as a row of small dashes. If someone knows the secret, I'd love to learn it.
I'm posting this reply on the Forum rather than sending it by email. I don't mean to be critical or rude, but, unless it is a confidential issue, requesting private email answers deprives other Forum visitors of the chance to learn something and perhaps to disagree.
Also, I'm not sure if situation is the same in Australia, but many people in the US feel that revealing their personal email addresses in a public forum exposes them to unwanted messages from dubious sources. Not that anyone on this board is dubious, but you never know who's lurking....