have people be able to send users messages via email. You could have an opt in option where people can email you via a web form. It would tell you the person that emailed you, and the message. If the user doesn't want that, they don't enable the feature. 

Edit: Didn’t have my coffee when I first read this. The post seems to be about user to user communication and not developer to user. Feel free to ignore this.

This is already a thing. If you go to “Edit Game” -> “Interact”, there will be an option to e-mail all people that have bought your game. Each e-mail has to go through admin review, in order to avoid spam.

Users already have the option to disable such e-mails from games in their settings.

I think this is asking about the other way around, a link on a developer's profile that a user can use to DM the developer.