Reservations are totally separate from the app and there are often better places to make the reservations than the Eurail website. I'll add some more information about that in a separate reply.

You should make the reservations for the 4 of you at the same time. 

The recommendation from the experienced travellers in the community is to only make reservations through the Eurail website if no other options are available. Please read more about that below. 

Here is some useful information from the experienced travellers in the Community regarding both planning, reservations and activation of pass and travel days. 

 Planning

The rail planner is normally not up to date, as it only is updated once a month, so to be sure of the time table you better check the timetable and availability on the websites of the national railways. The bigger national railways, like DB (Germany) SBB (Switzerland) and ÖBB (Austria) cover several countries. 

 Reservations 

There is a limited number of pass holder seats on the Eurostar between London and mainland Europe. On popular departures and during high season those sell out weeks, and sometimes months in advance.

The best place to see the availability of passholder seats and make reservations on the Eurostar is 

https://www.b-europe.com/EN/Booking/Pass#TravelWish

If you don't get any result, press "later trains" and eventually you will find the next available connection. 

If you have a mobile pass you need to generate a Pass Cover Number in order to make the reservation at b-europe. You do that here in the PCN generator:

https://community.eurail.com/news-and-announcements-39/pass-cover-number-generator-is-live-5653

Thank you @AnnaB  for the quick response.

Would you help me understand why all 4 family members that travel together cannot be linked to the same trip?

Secondly, when I make seat reservations, do I have to have the passes activated for that day?

They can only share the itinerary in "My Trip". Once you've completed the broad outline of your trip on one pass, you can press the three dots, on the top of the screen and select "share as link". With this link, everyone can get the itinerary. Note thzt this are not linked, so changed made after the link has been shared, will not be able to come through.

Note, we generally recommend to let each person have it's pass on his/hers own phone. If one phone gets lost, it's easy to conjur a pass up on an other device. But it's more difficult if the phone with 4 passes on get's lost. It's also a bit more practical with ticket checks, are scanning through gates, each having the ticket by hand. 

If you make seat reservations for 4 people at the same time in the reservation process, you'll be sitting together. Passes do not need to be activated, when making reservations. Reservations are not linked to the app what so ever. Advice is only to activate the pass until just before travelling, same for activating travel days. 

Passes need to be activated and have a valid travel day when you board trains of course. The pass is your ticket. With trains with mandatory reservations, like Eurostar, you need a valid ticket and seat reservation, before being able to board. 

Thank you @BrendanDB  for your reply.

Can you help me understand the order of the process?

1) Add 4 passes to my mobile phone (done)

2) Create an empty “Family Trip in Europe 2023 - Passenger 1”

3) Make 3 copies of the above trip for the rest of the 3 family members.

So, at this time, my phone has 4 empty “Trips” and each trip is linked to a family member’s Eurail pass.

What are the next steps?

4) Search for journeys and add them to all 4 trips. So gradually, the trip will contain all journeys we intend to take

5) Wait till 1 day before the 1st travel date and activate the 4 passes on my mobile
6) Generate ticket QR codes on the travel day
 

That's about it, for step 4 and 5 I can recommend this extra:

For step 4) You can search for journeys and add them on one trip of one person. Once you are done with all the trips (see that as your travel diary), you can share that trip as a link. With that link the three other get imported the same journeys on their trip, so you don't have to add all the journeys separately. 

5) Best even wait until the travel day itself to activate it.

Thanks @BrendanDB 

Our children are 14 and 15, and I worry that they will lose their phones during the trip. So I intend to use my phone for all Eurail ticket purposes. Do you meant to say I don’t need to create 4 trips until the first travel day? Just keep one trip and add journeys and 1 day before the 1st travel day, make duplicates and assign to each family member?

Can the Eurail pass be activated on the same day of travel? I thought it has to be activated a day prior. Thanks for letting me know.

As you have created 4 trips on the same phone it is easy to add the trip to all 4 Trips from the planner or after manual addition. Once you identify your chosen train you simply select “add it to trip”. The app will prompt you which trips you want to add it to, at which point you simply select all 4 and click continue.

You do not need to activate the passes for any of this so simply activate each individually and add the train from each trip to its own pass - that does require 4 operations.

I suspect yoy will need to remove trains from the trips individually, but as long as they haven’t been transferred to the pass this is simply a housekeeping chore.

For clarification your pass becomes a valid ticket for travel once each train you travel on is added from your trip to your pass. You will see all valid trains for that travel day listed below the QR code.
 

The list is dynamic and as you add or remove trains you have activated the list will be updated. So if you have transferred a train from your trip but then you change your mind for any reason you simply remove the train from your pass by reversing the slider. Then add an alternative to your Trip and then operating the slider to transfer it to your pass. The first train is removed and the new train appears.

Thank you @Yorkie  for your assistance.

I have a follow up question. Suppose we have set up the trip to take 2 trains a day (that do not need reservations). After taking the 1st train, we want to take a longer break and take a different train than what is in the “trip”. Can we change the 2nd train like that?

In other words, once a day is marked as a travelling day, do we get the same QR code for all train rides in that day? Or, does each train ride get a different QR code?

What happens if we miss a connecting train? In that case, can we generate a QR code for later train ride without having to pay extra?

The QR code is the same for the whole day, but the train you travel with has to be added as a journey.

If you add each train as a separate journey it is easy to delete and add trains when your travel plans change.

Just for confirmation of Anna’s post, you can indeed change your mind at any time and add or delete trains on the fly. As long as your new train is shown below the QR code, along with any earlier trains you travelled on, then you are fine. You can even decide to leave a train at any station on route, and make a later train by adding it to your trip and transferring to your pass. Just remember you might need a reservation for new trains - easily checked at the station.

Thank you @Yorkie and @AnnaB  for your detailed responses

Hi @AnnaB ,

Would you please clarify this technique? Do you mean to say it is better to add each train ride as a separate journey?

Is it possible to link more than one journey to one Eurail pass?