A new settings is added in the options to let you to set who will be able to send you notifications. By default, it’s set to “Your Extended Circles”, this means that notifications will pop up every time everyone in your circles and in their circles +mentions you, share something with you, or invites you for a game or just hangout.
For most people, this probably will be fine. However, if you have a large number of people in your circles, it’s easy to be flooded with these messages. You can change the setting to only your circles, only you, or only from your custom circles. There’s no one size fits all solution, and the choice is yours.