One of the most annoying features of WordPress Multisites (besides domain mapping) is adding a Super Admin a few years after your network is established and grown. Sure, being a Super Admin allows said user to access any site on the network, but the user is technically not added to the site – so the Super Admins users My Sites tab will only have sites that s/he has been manually added to. The user is then forced to jump between sites with a headache of manually typing.
The follow script solves the problem, it runs through all super admins and adds them to all sites in the network they’re not yet added to.
The function needs to be run once to get everyone up to date. For the future it can be wrapped in a $_GET conditional to fire manually, a cronjob to run in background, or like I did, a transient, so that you can run it as often as you need. Mind you this isn’t a network plugin or intended to run on all sites, it just needs to run once on a single site. All the multisite data and blog_switching annoyance happens within the functions themselves.