Oh, no! Have I missed Minyan?
The mitzvah tefillah b’tzibbur (communal prayer) consists of davening (praying) with a minyan (quorum of at least ten men), with the minimum of six participants who are actively davening. To fulfill his obligation of tefillah b’tzibbur, each active participant should daven the essential part of tefillah—the silent Amidah (“standing prayer,” colloquially called Shemonah Esreh) with all the others, beginning the first of the nineteen brachos of the Amidah with the congregation —or according to some opinions, while they are still reciting the first three brachos (which can be considered as one brachah).
In extenuating circumstances, there are more lenient poskim to rely on: Some maintain that as long as a participant begins the Amidah before the congregation has concluded it, he is considered to have davened with a minyan; still others consider participation sufficient at the time the shliach tzibbur (the prayer leader, the chazzan) begins his repetition of the Amidah, or even at any point in his recitation. Although these options aren't considered tefillah with a minyan in the optimal sense, each contains some aspect of tefillah b’tzibbur.