You can click on a circle from your Circles page, click "share" and share it with others. When you do this, you're only sharing its members at that exact time, and the circles name is always private to you. Other changes you make afterwards are also private.
"One of my favorite parts about circles is how they help me control who I share with, as well as what I read," says Prater. "In fact, many of you have created lots of great circles around topics that interest you (like Photographers) to bring lots of great content to your stream. In these cases, we've heard that you actually want to share your circles with others. Both to save your friends some time, and to connect them with interesting people and content."
"We hope this new feature helps you share and find lots of great content in Google+, while still giving you important controls over how you read and share," he says.
This should make Google+ a better content discovery service, because users will be able to discover new people that share things they're interested in. It's worked pretty well with Twitter lists so far.