It's been mentioned, it has NOT been "covered".
Anyone know how to modify wp-signup.php or/and wpmu-functions.php to make it allow the creation of blogs at http://domain2/wp-signup.php in addition to http://domainwpmuoriginallywasinstalledon/wp-signup.php?
http://wpmudevorg.wordpress.com/project/Multi-Site-Manager is nice, it barely helps but it allows you to add multiple domains - without copying any of the settings, so you have to copy the tables for themes etc from the original site domain into the other domains. When that's done then it works, and you can (only) copy blogs from the original (first installed on) site's domain to any of the later added domains.
However, as said, if you sign up using one of the other domains then the blog is created as a blog on the first domain, so a mod / bugfix / something is needed to make it actually possible to sign up using later added domains.
And yes, I know there has been plenty of chicken little meaningless debates on how to do this before, as listed at
http://mu-plugins.livelyblog.com/HOWTO_host_multiple_domains_on_the_same_MU_installation
And all previous threads are all mickey mouse bullprop with nothing which solves anything.
Any idea how to allow people to sign up using other domains?
Any idea how to make a user logged into site2.com be able to comment etc as a logged in user at site1.com and site3.com?