[lotharrr]

I've been able to create URL-based "unlisted" hangouts with:

http://plus.google.com/hangouts/_

That will redirect you to a new hangout URL, and (in my experience) you can just copy/paste the new URL to other folks. To use it, they'll need to be logged into a G+ account, but it doesn't matter which one.

[julianz]

Oh upvote the shit out of this! The only hassle we've had with Hangouts is how to start one by yourself and then get a link that's easily pastable into IM so everyone else can join, and this works like a charm.