How we can get “get_post_meta” of specific user who added it

There are various ways to achieve this you can either generate metakey value for update_post_meta in such a way that it will contain user id.

//Example :
global $user_ID,$post_ID;
// Store the values
update_post_meta($post_ID, 'meta_key'.$user_ID, '$meta_value');
// Get the values
$meta_values = get_post_meta($post_ID, 'meta_key'.$user_ID);

or you can save it in user meta as following

//Example :
global $user_ID,$post_ID;
// Store the values
update_user_meta( $user_ID, 'meta_key'.$post_ID, 'meta_value' );
// Get the values
$meta_values = get_usermeta( $user_ID, 'meta_key'.$post_ID );