I don't think notifications will be sent when you add other people to your trip.

What do you mean by connecting train journey to your trip on the website? You normally do that in the app (if you have a mobile pass), before boarding a train. On the website, adding people to your trip is only useful for making reservations together.

I want to add a train journey to my pass as well as others and I would like to make reservations together. I think I have figured out how now, but the app is really buggy.

*website

You have to do that in the app. You can only do that for yourself, not for others.

You don't need to do this to be able to make reservations.

You can do that on the Interrail website (the app also forwards you there), or elsewhere, depending on the train or train company.