Hello and welcome to the forums [member=ahmed fathi].
1. I would modify the list_template.php under assets/grocery_crud/themes/your_theme/views the html code for the button
2. I would make an ajax call to a controller function that would pass the selected emails and send to these only (the email message)
For the part of adding a way to multiple select I would use the trick of the fake column (just add on the columns function an extra field) and then I would use the callback_column to make an html checkbox to this column for every row/record of the table.
Of course along with all these you need some jQuery magic to get the checked checkboxes and pass them to the ajax call of the controller function that actually sends the email but I think this is pretty easy.