This Halacha is an excerpt from our Sefer
May Chazaras Hashatz be recited if only five people Davened together?
In general, a Minyan requires ten people to be in the room, and six people to have Davened together Sheoneh Esrei in order to recite Chazaras Hashatz. Nevertheless, in the event that in a Minyan [of ten or more people], there was less than six people who Davened Shemoneh Esrei together, it is nevertheless permitted for the Chazan to recite the repetition for the sake of the Kedusha. (Nevertheless, it is not considered that the congregation has Davened with a Minyan, and the above repetition does not have the status of a regular Chazaras Hashatz.) However, initially in such a case, instead of Davening Shemoneh Esrei in silent and then repeating Shemoneh Esrei for the sake of the Kedusha, the Chazan is to Daven his silent Shemoneh Esrei aloud until after Kedusha and then recite the remainder of Shemoneh Esrei in silent. [Accordingly, even in the event that one realized that there were only five people after finishing the silent Shemoneh Esrei, if possible, it is better for one of the other people who have not yet begun Shemoneh Esrei to say it aloud until Kedusha, and then say the remainder silently, than to have the Chazan repeat it. If everyone in the room already heard Kedusha, then the Shemoneh Esrei may never be repeated aloud unless six people Davened together.]
 Admur 69:4-5; Piskeiy Teshuvos 69:1
 Admur 69:4; Michaber 69:1; Tehila Ledavid 69:2; Ashel Avraham Butchach 69; Imrei Yosher 2:9; Afrasakta Deanya 2:16; Minchas Yitzchak 1:57 and 10:107
Other opinions: Some Poskim question if according to the Radbaz 4:241, brought in M”A 69:4, one may recite the repetition for the sake of the Kedusha in such a case. [Biur Halacha 69:1 “Omer”]
 See Admur 69:5 in parentheses
 Admur 69:5; M”A 69:4; Piskeiy Teshuvos 69:1