This Google Script saves Gmail attachments to Drive and shares the saved files with users based on rules defined in the message body itself. Here’s a sample rule:

# view@example.com, v #
# comment2@example.com, comment@example.com, c#
# edit1@example.com, edit2@example.com, e #

In the above case, the file(s) would be shared with all these users but the sharing permissions would be different. The user view@example can only view (v) the document, the users edit@example have editing permissions while others have commenting permissions.

function ShareDriveFiles() {
  var threads = GmailApp.search('subject:#share');

  for (var t in threads) {
    var messages = threads[t].getMessages();

    for (var m in messages) {
      var attachments = messages[m].getAttachments();

      if (attachments.length) {
        var body = messages[m].getPlainBody().replace(/\s*/g, '').toLowerCase();

        for (var a in attachments) {
          var rules = body.match(/(#[^#]+,[e|v|c]#)/g);

          if (rules) {
            var file = DriveApp.createFile(attachments[a]);

            for (var r in rules) {
              var line = rules[r].substr(1, rules[r].length - 2).split(',');
              for (var s = 0; s < line.length - 1; s++) {
                switch (line[line.length - 1]) {
                  case 'e':
                    file.addEditor(line[s]);
                    break;
                  case 'v':
                    file.addViewer(line[s]);
                    break;
                  case 'c':
                    file.addCommenter(line[s]);
                    break;
                  default:
                    break;
                }
              }
            }
          }
        }
      }
    }
  }
}