How 'bout using a content management system such as Mambo - people can register themselves + you can approve them for access.
Gives you loads of options re features such as guestbooks/forums/document handling/downloading of files/calendar etc....and good community support too.
http://www.opensourcecms.com/ has a list of CMSs - some more difficult than others (stay away from typo3 unless you want an ulcer!)
Mambo lives here:
http://www.mamboserver.com/ with loads of mods here http:
www.mamboportal.com and support here:
http://www.mambers.com/
Hope this helps,
Cheers
Charles