What is the law if there wasn’t a Minyan in the room during Shemoneh Esrei-May Chazaras Hashatz be recited upon the arrival of a tenth man?
If there wasn’t a Minyan in the room during Shemoneh Esrei and a tenth man arrived prior to Chazaras Hashatz, then so long as six people Davened Shemoneh Esrei simultaneously, Chazaras Hashatz may be recited. One may even initially Daven Shemoneh Esrei with less than a Minyan and then bring a Minyan into the room for Chazaras Hashatz. [Nevertheless, in such a case, it is not considered that one Davened with a Minyan, and it is only regarding Chazaras Hashatz that we permit its recital based on the fact six people Davened simultaneously.]
 Kaf Hachaim 69/1; Shnos Chaim 6/14 in name of Kesher Gudal 18 [did not find in Kesher Gudal]; Minchas Yitzchak 10/107; Peri Hasadeh 2/97; Imrei Yosher 2/9; Maharshag 1/9; Piskeiy Teshuvos 69/1; No mention is made in Admur 69/4 that there must be ten people in the room during Shemoneh Esrei in order to say Chazaras Hashatz
Other opinions: Some Poskim rule one must have a Minyan present during the silent Shemoneh Esrei in order to recite Chazaras Hashatz. [Rav Poalim 2/26; Ben Ish Chaiy Teruma 16; See Minchas Elazar 2/76; Har Tzevi 1/52; Tzitz Eliezer 14/6] This ruling applies specifically according to the Radbaz 1/241, brought in M”A 69/4, that if everyone Davened Beyechidus, they can no longer have Kaddish and Kedusha recited, and hence those who suspect for the opinion of the Radbaz must be stringent. [Poskim ibid] Admur 69/4 omits the opinion of the Radbaz.
 Imrei Yosher ibid; Minchas Yitzchak ibid; See Admur 69/4-5