Okay, problem resolved. I found that in my “Network Options”, registration was set to disabled. However, even changing that to User accounts may be registered still has not had the effect of enabling the Membership option: it is simply enabled by default, and I don’t seem able to disable it for specific blogs. Is this intended behavior?
Okay, I’ve got the same problem, and in checking out my database, I’ve dutifully set wp_capabilities to a:1:{s:13:"administrator";b:1;} as suggested, without effect. I’m running a multisite, so I also set wp_4_capabilities, wp_7_capabilities and wp_8_capabilities to a:1:{s:13:”administrator”;b:1;}
A screenshot of the relevant rows is at http://www.webgeekstress.com/images/database.png
Ideas?