BuddyPress.org

Try this.

function redirect_to_forum() {
global $bp;

$path = clean_url( $_SERVER );

$path = apply_filters( 'bp_uri', $path );

if ( bp_is_group_home() && strpos( $path, $bp->bp_options_nav ) === false )
bp_core_redirect( $path . $bp->bp_options_nav . '/' );
}
add_action( 'wp', 'redirect_to_forum' );