The man entered by following behind when someone opened the door with a card.
The man entered by quietly following behind as the door was opened with a card.
ご質問ありがとうございます。
・「The man entered by following behind when someone opened the door with a card.」
「The man entered by quietly following behind as the door was opened with a card.」
（意味)その男はその人がカードをかざして入口のドアを開けたとき、後ろについて行くことで侵入した。
<例文>How did he enter? //The man entered by quietly following behind as the door was opened with a card.
<訳> 彼はどうやって侵入したの？//その男はその人がカードをかざして入口のドアを開けたとき、後ろについて行くことで侵入した。
参考になれば幸いです。
Some suspicious person came up behind someone who lives in the apartment building and went into the building after him.
この場合、次のような言い方ができますよ。
ーSome suspicious person came up behind someone who lives in the apartment building and went into the building after him.
「不審者がそのマンションに住む人の後ろに近づいてきて、（マンションの住人の）後から建物に侵入した」
suspicious person「不審者」
to come up「近づく」
ご参考まで！
アメリカ合衆国 
カナダ 
