Topic speaks for itself.
I know you can set the group of which registered users join on signing up.

However I'm wishing to change this via the site method (php way)

What I'm wanting to is run multiple sites off the one database but each site is different.
So basically different site with different permissions but same database, users etc...

Can anyone point me to the right PHP files or help me build a script to make this progress possible?

Thanks!