Glad the issue could be replicated.
@elangley: thanks for submitting a trac ticket. I’m definitely interested in having this issue fixed. And thx as well for the advice on the BuddyPress Import plugin, will try it.
Just a long shot… do you use an e-mail address as username?
Have a look at my question https://buddypress.org/support/topic/problem-with-email-adress-as-username/ from some months ago. Unfortunately nobody replied.
Workaround is to to create usernames without @ (we always create users ourselves, completely private groups, members need to be invited).
Didn’t check recently if the problem has been resolved or not.
Installed a plugin for users to login with their e-mail address or their username (which they typically forget) Plugin is WP Email Login: *Allows you to log into WordPress using your email address instead of a(nother) username.* Can be found at https://wordpress.org/plugins/wp-email-login/