Did you get a notification that someone wants to connect with you on Google+? That means you've been added to a circle. Anyone can add you to one of their circles without your confirmation. If someone adds you to a circle, that person isn't automatically added to one of yours. But because you'll often know the person who adds you to a circle, you can easily place them in a circle as well (only if you want to).
When someone adds you to a circle, it won't give them additional access to your profile information. It may however make it easier for that person to take actions that generate notifications for you--like commenting on your posts or sharing with you directly. You can always adjust your notification settings if you're receiving too many notifications.
If you want to limit someone from interacting with you on Google+, you can block that person.
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