If you still have access to the user account email, simply reset the user email and log in into his HipChat account and transfer the owner to yourself.
However, if you don't have access to the user email, simply reach out to support to help you transfer the ownership of your HipChat group. Do note that only existing admin can request an owner transfer and normal user are not accepted.
Hope this helps.
...have mention name FirstnameLastname. Fe.: @MaartenCautreels This certainly helps to know who is being mentioned in a conversation. Imagine your colleague sends the following message...
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