They want to
    conceal their marriage
Question

                I am converting to Islam, however I find it
      difficult to tell my family..I will do so in time however is it necessary to announce it
      officially. 
  I wish to also become a wife to a man (who already has a wife and
      children) for islamic reasons. The difficult part is also telling his family
 Is it necessary to annouce our marriage? As we do not wish to hurt anyone.
 I trust you will be able to guide me as will Allah.
Praise be to Allah.You do not have to announce your Islam, whether officially or to your
    family, and it is permissible to keep it a secret if there is some fear of harm. As for
    marriage, however, the sunnah is to announce it, because the Prophet 
 (peace and
    blessings of Allaah be upon him) said: “Announce marriages.” (Reported by
    Imaam Ahmad in Musnad ‘Abd-Allaah ibn al-Zubayr; classed as hasan in Saheeh
    al-Jaami’, 1072). If a marriage is announced in one city or
    location, it need not be announced in another, but we do not advise you to conceal the
    news of your marriage, especially since such news usually becomes known eventually. This
    news is likely to be a big shock to the first wife, so announcing the marriage is in the
    interests of all concerned, as it establishes fair and just treatment for both wives. And
    Allaah is the Source of strength.