BuddyPress.org

@robert, If this happened just recently, there is a high chance that there is a conflicting plugin. It is usually not the php file that is incorrect. First, try deactivating all plugins that relate to buddypress, and see if the problem persists. (deactivating will not delete settings)

If you have done that, and it is still not working, try installing the BP template pack again. This will recreate the template files, such a groups, into your custom theme again.