Hello Keith,
On my BuddyPress site, 3.9 beta doesn’t work at all. Once the plugin is activated, the activity log remains completely blank. Contrarily to version 3.8, new users can register, but so do the spammers :).
So I switched back to 3.7 which worked perfectly. I will try to get more details later, with 3.9 beta on a local copy of the site.
Thanks!
Stéphane