It sounds like you may have signed up using a different email (or potentially mistyped your email when signing up). One option to figure it out is to search your mail application for emails from HipChat and see what email they're addressed to.
If you continue to have problems, I'd suggest you contact support by going to https://support.atlassian.com/servicedesk/customer/portal/32. They should be able to help you figure out your issue. Having the name of the subdomain you created when signing up will also help them narrow things down for you.
...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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