Hi I was wondering the same thing and have been searching all over the web for a simple answer to the same question of the original poster. This article is great in the sense it addresses every aspect I can imagine for group chat and have learned how to use and understand group messages using this same article. Whenever I search on the web for specifcally how to rejoin a group message chat after clicking leave this conversation, every result always comes to the same source (or would reference it by saying the same thing). But I can't find anywhere how to simply "come back" after leaving. From the answer yougave and how you related it to the article, from what I understand (although if this is the case, it's not clearly put) is that you compose a new message to the same list of contacts that were in the group message chat that you had left. Is this the answer?
Also, if it is the answer I must assume it's a very rare case since the detailed support articles (which almost always have worked for me in the past many years) as well as the forums and blogs on the web regarding group messages especially specific to Yosemite and iOS 8, I don't see any reference of having an option to rejoin the same group you had left. This post was the first and only reference to this question.
