How do I add transactional notifications in my nodejs app

Hello fellow coders!
I am currently working on my college project and I am trying to figure my way around this notification thing. This project helps in tracking expenses and splitting bills.
At the current stage I have been able to create groups and add other friends using their email addresses but I a now want to work up on adding a notifications section where users get the updates if any new groups or transactions are added
ANy kind of help is appreciated
Thanks in Advanced

You would need to post a link to your full project code, so we can see exactly what you have written so far.