If you're using the macOS client, you can add multiple orgniazations to the client the directly (via File > Log In To Another Team). Unfortunately, this doesn't yet work for the Windows or Linux client, but you can always connect to another organization via the web client (by going to othercompany.hipchat.com/chat).
Also of note, to be added to another organization's HipChat account you do need to use a separate email. (This is often trivial if you use something like Gmail, which allows you to append suffixes to your email).
Another alternative is to use guest access, which is great for temporary discussions around specific topics
...from the beginning. We have built up a lot of content in HipChat, with it being a core tool in our distributed company model. While it is true that we didn’t need to move to Slack immediately, we felt it...
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