Link from Main Site back to logged in user's Dashboard in Multisite

When a user logs into my multisite install, they are automatically redirected to the main site (elevatesites.com), where there’s a training library.

On the menu of the main site, I want a link that says “DASHBOARD” that, when a user clicks on it, will send that user directly to the dashboard of THEIR blog (from the main site).

How can I achieve this?

Here’s the front page of the main site and the dashboard link:

https://screencast.com/t/koVEJ9jV

For a more in depth example,

When user “johnc” logs in, he’s taken to elevatesites.com (the training library)…

And when he clicks on the “DASHBOARD” link, he’s automatically sent to *HIS* blog: http://johnexample.elevatesites.com

The same would go for ANY user. When they log in, each user will be sent to their own Dashboard.