Have you tried this plugin? http://wordpress.org/plugins/welcome-pack/
Also I think this is the buddypress function that has a filter to change from functions.php
COPY CODE
function bp_get_email_subject( $args = array() ) {
	$r = wp_parse_args( $args, array(
		'before'  => '[',
		'after'   => ']',
		'default' => __( 'Community', 'buddypress' ),
		'text'    => ''
	) );
	$subject = $r['before'] . wp_specialchars_decode( bp_get_option( 'blogname', $r['default'] ), ENT_QUOTES ) . $r['after'] . ' ' . $r['text'];
	return apply_filters( 'bp_get_email_subject', $subject, $r );
}
 
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