Re: WPMU, require login to view any part of site, but allow registration
Try “registered users 2” ( https://wordpress.org/extend/plugins/registered-users-only-2/ ). It is compatible with MU and buddypress. I use it for one of my sites. “Authenticate” looks like it is very very old (only tested compatible up to wp 2.5.2)
Sorry I can’t help more 
