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

this is the project, Can you also review it? while you are at it, this is my first full stack project and I am struggling a lot