If one began saying Kaddish (or Kedusha, Chazoras Hashatz, Nesias Kapayim, Krias Hatorah, or Haftorah) with the requisite minyan of ten, and then one or more of the ten left, one may complete the Kaddish (or the other above mentioned mitzvos) as long as there are at least six men remaining (Shulchan Aruch w/Mishnah Berurah 55:2).
If the Kaddish was the one immediately preceding Borchu, one may even continue to say the Borchu with the remaining six men, since the Borchu is considered to be connected to the Kaddish that was started properly with a complete minyan of ten (Shulchan Aruch w/Mishnah Berurah 55:2).
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