The Halacha requires a Minyan in order for the Chazan to begin the Chazara of the Amida. This Minyan must consist minimally of the Chazan and nine other men who can answer Amen to the Berachot. Some people mistakenly think that having six people, a majority of a Minyan, is sufficient.
Unfortunately, some times the Chazan begins the Chazara with exactly a Minyan, and someone leaves in the middle, leaving the Tefila without a Minyan. In such a case, The Shulhan Aruch (55:3) rules that as long as a majority of the Minyan remains, the Chazan may continue to recite the Chazara. Any unit of Tefila that was started with a Minyan, may be concluded even without a Minyan. This principle applies even to the Kedusha, and the Chazan may recite the Kedusha without a Minyan. Even though the tenth man left before the Kedusha, it is considered one unit together with the Chazara. However, Birkat Kohanim and the Kadish Titkabel (which is connected to the Amida) are considered separate units, and may not be recited without a Minyan.
This Shulhan Aruch (55:2) also applies this principle to Kaddish. If the tenth man left the Minyan in the middle of the Kaddish, it may be concluded without a Minyan.
Even though the Tefila may be concluded without the missing person, clearly, one should make every effort to remain in the Minyan and not leave before the end of the Tefila.
Any unit of Tefila begun with a Minyan, may be concluded without a Minyan, as long as the majority remains.