Currently all multi-person chats in HipChat must occur in rooms. However, we've had a few suggestions to support ad-hoc chats without the overhead of creating a room. For example: https://jira.atlassian.com/browse/HCPUB-1856. You can go to that issue and vote / watch / comment in order to receive updates.
If you're just looking to have a place where people can enter and exit without being a "permament" room member, then it sounds like a public room might work for your purposes. You can quickly create one by using the keyboard shortcut CMD-SHIFT-N and then invite whoever you want to chat with with CMD-I. It's not quite the same, since the room requires a name, but it might help remove some of the overhead and get you close.
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