The File.addViewer or File.addEditor methods of Google Apps Script allow you to share files in Google Drive programmatically. These methods will however always send an email notification to the person with whom the file is being shared.

If you would like to share a file in Google Drive silently without sending them an email notification, the advanced Google Drive API can help. The Permissions.insert method lets you specify sharing permissions for a file. The role can be either owner, writer or editor. The additionalRoles[] should be sent to commenter to add someone as a commenter.

When calling Drive.Permissions.insert, specify the advanced option “sendNotificationEmails” to “false”. You’ll have to enable this however in your Google Apps Script project.

 Drive.Permissions.insert(
   {
     'role': 'writer',
     'type': 'user',
     'value': 'bob@example.com'
   },
   fileId,
   {
     'sendNotificationEmails': 'false'
   });

The method can be used to disable email notification when sharing folders too.

var folderid = folder.getId();

folder.setSharing(accessinfo, permissioninfo);

if (editors.length > 0) {
  for (var i = 0; i < editorsemails.length; i++) {
    Drive.Permissions.insert(
      {
        role: 'writer',
        type: 'user',
        value: editorsemails[i],
      },
      folderid,
      {
        sendNotificationEmails: 'false',
      }
    );
  }
}

if (viewers.length > 0) {
  for (var i = 0; i < viewersemails.length; i++) {
    Drive.Permissions.insert(
      {
        role: 'reader',
        type: 'user',
        value: viewersemails[i],
      },
      folderid,
      {
        sendNotificationEmails: 'false',
      }
    );
  }
}