HipChat accounts are specific to the group that they're a member of. In order to invite a user into your group, you will need to send them an invite specifically (adding users can be accomplished at https://hipchat.com/admin/users). You can also let users sign up via a special URL that can be found at https://hipchat.com/admin. Users will need to use a separate email for each account they want to connect to HipChat with.
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