BuddyPress.org

…shown up as members in BP, and some have not.

You mean don’t show up on the Members page?
Can you go to the profile page of those members?
ala – yoursite.com/members/bradley/

If they’ve logged in, they should have a last_activity entry – without which they won’t appear on the Members page.

If you aren’t concerned about existing last_activity entries for those who currently do appear, paste this function in your theme/functions.php.
It will create a last_activity entry for all users and set it to the current time.

Load the site once and then remove this function so that it does not run again.

function brad_add_last_activity() {
    global $wpdb;
    foreach ( $wpdb->get_col( "SELECT ID FROM $wpdb->users" ) as $user_id ) {
        bp_update_user_last_activity( $user_id, bp_core_current_time() );
    }
}
add_action('bp_init', 'brad_add_last_activity' );