He recited quietly at the beginning of his prayer, then someone else joined him; should he complete it reciting quietly or out loud?
Question

                I missed the jamaa‘ah (congregational prayer) for ‘Isha’, so I started to offer the prayer in the mosque, and in the first rak‘ah, after I had recited al-Faatihah and started to recite a short surah, someone else joined me so as to pray with me in jamaa‘ah. What should I do? Please note that I was praying quietly and I was not reciting out loud.

Should I recite out loud after this person joined me? Should I start al-Faatihah all over again or should I complete my recitation as if no one else had joined me in my prayer?
Praise be to Allah.
      Reciting out loud in prayer is a Sunnah for the imam. As for the one who 
      is praying on his own, he has the choice of reciting out loud or quietly. 

    Ibn Qudaamah (may Allah have mercy on him) said:

    Reciting out loud is prescribed for the imam, and it is not prescribed for 
    the one who is praying behind an imam, and there is no scholarly difference 
    of opinion on this point. That is because the one who is praying behind an 
    imam is enjoined to listen attentively to the imam. In fact he is not 
    allowed to recite, so that he may do that (listen attentively). In the case 
    of one who is praying on his own, what may be understood from the reports 
    narrated from Ahmad is that he has the choice. The same applies to one 
    missed part of the prayer with the imam, so he gets up to make up what he 
    missed. Al-Athram said: I said to Abu ‘Abdullah: A man missed one rak‘ah of 
    Maghrib or ‘Isha’ with the imam, so he got up to make it up; should he 
    recite out loud or quietly? He said: If he wishes, he may recite out loud, 
    and if he wishes, he may recite quietly. Then he said: Reciting out loud is 
    only for the jamaa‘ah (congregational prayer). I said to him: Does the same 
    apply if he is praying Maghrib or ‘Isha’ on his own: if he wishes he may 
    recite out loud and if he wishes he may not do so? He said: Yes; Reciting 
    out loud is only for the jamaa‘ah (congregational prayer).

    End quote from al-Mughni (1/333-334) 

    Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said: Reciting out loud 
    in a prayer in which the recitation is to be done out loud is not 
    obligatory; rather it is what is preferable. If a person recites quietly in 
    a prayer in which it is prescribed to recite out loud, his prayer does not 
    become invalid, because the Prophet (blessings and peace of Allah be upon 
    him) said: “There is no prayer for the one who does not recite the Essence 
    of the Qur’an (al-Faatihah)”, and he did not stipulate whether this 
    recitation is to be done out loud or quietly. So if a person recites what he 
    is required to recite, either quietly or out loud, then he has done what is 
    required. But it is preferable to recite out loud in prayers in which it is 
    Sunnah to recite out loud and in which doing so is customary, such as Fajr 
    and Jumu‘ah prayer. 
If 
    a person deliberately does not recite out loud when he is leading the 
    prayer, then his prayer is valid, but it is lacking. 
In 
    the case of one who is praying on his own, if he offers a prayer in which 
    recitation is done out loud, then he has the choice between reciting out 
    loud or quietly. So he should decide what will be more effective in 
    energising him and enabling him to focus properly, then do that.

    End quote from Majmoo‘ Fataawa Ibn ‘Uthaymeen (13/73-74) 

    Based on that, if someone joins you when you are praying on your own, then 
    the Sunnah in this case is to recite out loud, because you became an imam 
    when that person joined you. But in this case you do not have to recite al-Faatihah 
    from the beginning; rather you should complete your recitation as if you 
    were on your own. 
If 
    you did not recite out loud until after you had completed al-Faatihah or had 
    completed the rak‘ah, or you did not recite out loud in this prayer of yours 
    at all, there is no blame on you, and the prayer is valid. 

    And Allah knows best.