I've done that a fair amount - there is actually a hack for it (two really, one requires a login, the other uses an authentication script.)
The first hack is
https://vborg.vbsupport.ru/showthrea...threadid=23524
And an add-on at
https://vborg.vbsupport.ru/showthrea...threadid=30642
It's amazing what you can do with these - I've got a login form on the homepage, and depending on your usergroup, the homepage is different for each group after they login.
For the built-in authentication, do a search for authvb.php