I am managing 100+ WordPress sites in a single multisite install. The sites are completely independent (i.e. the site's owners do not know that they are hosted on the same server). So far the multisite has served me well in maintainability (just one server to access to, one WordPress install, a single copy of each plugin, ...).
There are problems, though: with multisite the user base is shared between all sites (it's a network, after all), so that if a user sign up on a site, she can't sign up on another site with the same email; and I need the site admins to not know that they are inside a multisite.
Is there a way to make the multisite user register with the same email on different sites?
If not what's the best way to manage this use case?