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
Badges are a great way to show off community activity, whether you’re a newbie or a Champion.Learn more
Heya, Hipchat friends! We’re so happy you’re checking out Stride. Whether you know it or not, you have been instrumental in making Stride come to life. Every feature, design, and functionality...
Connect with like-minded Atlassian users at free events near you!Find a group
Connect with like-minded Atlassian users at free events near you!
Unfortunately there are no AUG chapters near you at the moment.Start an AUG
You're one step closer to meeting fellow Atlassian users at your local meet up. Learn more about AUGs