WordPress MU

I have a membership site on a separate server than where my Wordpress MU is hosted (everyone has a name/password to get in) and I'd like to provide an MU blog to those people only.

What would be the best way to allow access to those members without allowing the general public to sign up for a blog? Maybe:

1) Password-protect wp-signup.php ? (how?)

2) Only allow wp-signup.php if they clicked through from a specific page from the other website? (How?)

3) Something else?

Please let me know what the best solution would be, thanks.

An invite plugin, perhaps? Or import the user/passwords into the MU site, then check the box to allow only current users to create blogs.

LDAP

Thanks for the suggestions so far:

Andrea_r, the only "invite" plugin I could find said it didn't work w/the newest version of Wordpress MU... Are you aware of one that works?

Also, is there any existing way to import user/passwords into MU? (Is there a way to do it, or a plugin to do it?

kgraeme: I got a Fatal Error message when I tried to activate LDAP, and I can't find any documentation for it anywhere, can't even find a description of what it exactly does or how it works.

http://wpmudevorg.wordpress.com/project/wp-invites
http://wpmudevorg.wordpress.com/project/secure-invitations

There's two.

thank you for the suggestion andrea_r