When you use a pass and request reservations the system tries to put all people in the reservation together, but there is no guarantee, Also it is normally possible to seat 4 in a group at a table but if there are more than 4 (e.g. 5) the system struggles to know whether to split 4 +1 or 3 + 2 so you need to split your bookings into groups as you would like them seated to have any chance.

This is so for the majority of operators and booking sites.

There has been a recent thread on this subject.

If you make the seat reservation together you are seated together if possible. 

There is a limited number of pass holder seats on the Eurostar and 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

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

If you log in to your Eurostar booking on the eurostar.com site with the booking references on the ticket you have (at least for those booked through SNCB, if I remember correctly) I was able to change the seats that I had been assigned.