Manual code not working

As said in other review, it’s impossible to enter manually a code since they got saved in lower case while the are entered in upper case…

The fix: Modify the plugin -> locate function create_invitation_code in back-end.php and replace $code=... by
$code = strtoupper(sanitize_key( $code ));

Now the code you create are correctly saved in upper case but there I already saw other bugs (code deletion not working well)