What were Henryk, Halina and others chosen for?

Question

At Warsaw Ghetto, policemen randomly picked some Jews and the rest were sent to station for the extermination camp.

I assume some were picked because they were better built as later said by somebody that those present at the station were children, women and old people.

But then why the chosen ones were later escorted to the station? Halina says: "I wanted to be with you." and Szpilman comments: "Stupid." That makes me think as if Halina and others chose to come to the station.

But if it was so, then why were they escorted by a jeep? Surely, they weren't any privileged slaves.

And even a scene later, when they were about to board the train, Szpilman told Halina: "Wish I knew you better." I thought he got impressed that his sister sacrificed her life to be with the family, but then how can so many people be so selfless that they deliberately came to the station. I am surely missing something.

Any clarification would be welcome.

Answer 1

Szpilmans were forced to come to Umschlagplatz (the train station) because of the mass deportation from ghetto, called Grossaktion Warsaw:

Initially the roundups for the Umschlagplatz were supervised by the Jewish Ghetto Police. Houses or entire blocks were cordoned off and then all the inhabitants were forced to gather in a controlled spot, such as a closed off street or a tenement's courtyard. After a check of documents, the individuals were forced, under escort, to proceed to the Umschlagplatz. Emptied buildings were searched and those found hiding were either killed on the spot or joined with those proceeding to the square. Some young men were moved to the so-called Dulag, an internment camp, from which some were sent to labor camps rather than death camps.

Basically the Jews came willingly because it was either this or death. Yes, they were escorted - it was necessary means to keep order in these organised actions and possibly also to avoid people hiding in already emptied buildings.

Regarding Henryk and Halina they were most likely chosen for Dulag:

Some young men were moved to the so-called Dulag, an internment camp, from which some were sent to labor camps rather than death camps.

Your impression was correct, they came willingly to be with their family - from script:

           A loud cry from Mother. Szpilman and Father spin round.

                                 MOTHER
                     Henryk!

                                 REGINA
                          (glancing up, shocked)
                     Oh my God!

           Near the gates, among a large bunch of new arrivals, Henryk 
           and Halina.

                                 MOTHER
                     Halina! Henryk!

           Regina and Szpilman also call and wave. Henryk and Halina 
           struggle through to them. Halina falls into Mother's arms 
           and they hug.

                                 HALINA
                     We heard you were here...we...didn't 
                     want...we...we wanted to be with 
                     you.

           Mother comforts her. And so does Regina. Father smiles 
           sadly.

                                 SZPILMAN
                          (shakes his head, 
                          almost to himself, 
                          a forlorn smile)
                     Stupid, stupid!