Single Sign On to all sites in my entire WP Network.

Hello. New member and first support post.

I've got a wordpress network set up that has 4 controlled sub-sites. We will be adding about 15 more in the next 30 days. These sites are created and managed by my team. Users can log in to post content, but we are not allowing them to create their own sites. The whole network of sites is running great on the latest version of wordpress with multisite enabled. We are using the Domain Mapping plugin to successfully map individual domains to each subsite.

Like others in the forums, I made the mistake of thinking that the Domain Mapping plugin would solve my issue with being able to log in and out of all the sites on the network at once.

Come to find out, the cross domain cookies feature is only keeping a user logged into one sub-site and the main site. And this only seems to work if the user logs in at the sub-site. So if you log in at a subsite of the primary site.com called a.site.com or site.com/a which is mapped to sitea.com domain, you will be logged into all three.

For the confused, a picture tells a thousand words:
https://docs.google.com/drawings/d/1DnDwqWHxYqa6xHS4K7YFdiJA5ZuWZAogplaFsIGbleo/edit

Nice, but as the picture shows, doesn't quite do it for me.

I just need to be able to log in anywhere on the network, and be logged in on all network sites. envato is doing it. Log in to any envato.com site and then jump over to another one, and it still says "hello, ccrest" on all sites.

My network is asking me to log in on all sites. The accounts are there in the database, but logging in on each site in the network isn't what I need.

Can the single sign on feature of the Multi-domains plugin help me achieve what I need? Or have I misunderstood that plugin too.

What can I do to achieve this?