Have you taken a look at the iPhoto Library file (iPhoto '08+) in Macintosh HD / Users / YOU / Pictures / iPhoto / iPhoto Library via right clicking on it, selecting GET INFO and checking the Sharing and Permissions tab?
Btw, which Mac OS X and iPhoto version are you using?
I have the same problem. I can add myself and make it read & write but "everyone" is locked to read only and the only other option is "no access."
Edit: Figured out my problem. I had tried making a copy of a library in the shared folder. Apparently iPhoto doesn't play nice with multiple users yet. BOOOO.