do you have intense debate installed? I had the same problem after I converted my site to WPMU for BP. When I disabled Intense Debate it unlocked the settings page and you were able to save your changes, most importantly the one about enabling user registrations.
$user->id , I think this is the part that needs to be changed because this one references the id for comments or regular wp, I just don’t know what bp uses to reference the id. Does this make sense? Again, I’m just a beginner so if what I’m saying makes no sense at all please do let me know.