How do I send notifications for followers of a group when group content is updated? I want to build such functionality that allow the user to follow the other users, and when the followed person is updating the information, the follower can get notified for what they are doing.

I think to build the specified functionality, I can use the Flag module, but for the notification, what kind of modules are good to work with?
I am confused. There are too many modules like notification, or messaging; even the NodeJs module can notify.
Its depend on how regular you want to notify the user. If you want actual real time notification then go for nodejs module. See the documentation and you will get better idea about it.