Enable Private Messages in your Profile
You might have seen profiles have a Send Message button near the profile picture.
Here's how you can enable it in your profile. First, click on your Profile and click on Edit Profile.
Click on Contact Settings.
Select the first option to display the Send Message button and the second option to enable the Send an email option (this shows next to the Report this profile option).
If you want to restrict access to this option to just people in your circles or selected individuals, click on the drop-down and select the privacy level.
Note: Other Google+ users might still be able to mention you in posts even if you do not have this option enabled. If you want to restrict that, make the changes in the Google+ Settings page.