91. If the Minyan Loses Members

15:9 One should try to say the Kedusha in the blessing of Yotzeir Ohr with a minyan, though if that is not possible, it may be said alone.

15:10 If there is an exact minyan of 10 in shul, none of them are permitted to leave. If one does, the verse “Those who leave God will be destroyed” (Isaiah 1:28) applies to him. (Mishnah Brurah 55:12 says that one must remain until the current portion is finished; he need not remain to start the next section.) If a full minyan will be left, one may leave if he has already prayed with a minyan. If someone did leave when he wasn’t supposed to, those who remain may finish any part of prayer they have already started so long as the majority of the minyan remain (that is, six). So, for example, if the shaliach tzibbur started the repetition of the Shemoneh Esrei with ten, he may conclude it with Kedusha and everything else as per usual, even though one or two people walked out, leaving the minyan deficient. However, even though the shaliach tzibbur may say “Elokeinu v’Elokei avoseinu,” which is a stand-in for the blessing of the Kohanim, the Kohanim themselves may not get up to “duchen” without a minyan. Similarly, the shaliach tzibbur may not say the Kaddish that follows Shemoneh Esrei if the minyan is lacking members. That is because these prayers, while connected to the Shemoneh Esrei, are in fact separate from it. (Mishnah Brurah 55:18-19 says that we do say Kaddish in such a case.) Another example: if they started reading the Torah with a minyan and some of them left, they may complete the Torah reading but they may not add any additional aliyos or call anyone for maftir. In such a case, the one who received the last of the regular aliyos reads the haftarah and does not recite the blessings on it.