Let Users Log Into WordPress Using an Email Address or Username
Sometimes users don’t remember the username they chose when signing up for your WordPress site. However, they’re less likely to forget their own email address. You can give users the option to log in using either method by installing the WP Email Login plugin.
I actually tested this plugin on a Multisite / BuddyPress installation yesterday and it works perfectly. You can download the WP Email Login plugin for free from the WordPress plugin repository.