A looming Gmail feature will Google+ users simply type in a name to email each other. One does need to know the address. This applies to all on that social network and this particular “feature” is the default. 
For those that don’t want emails from everyone on Google+ one has to opt out by selecting “no one” or otherwise limiting this “feature” in a new Gmail setting.
Google says it will let users know of the change via Gmail when it is implemented.
Google Lessening Privacy Even More