Tzibbur

The Mishna Rosh Hashana (1:2) says בארבעה פרקים העולם נידון בפסח על התבואה בעצרת על פירות האילן בר"ה כל באי עולם עוברין לפניו כבני מרון שנאמר (תהלים לג, טו) היוצר יחד לבם המבין אל כל מעשיהם ובחג נידונין על המים.  Why does the Mishna switch from the word נידון to the word נידונין regarding water?  The Rogatchover explains based upon the Gemorah Sukkah (55b) that that the korbanos offered on Sukkos come to atone for the gentiles so that rain will come for them; they are also part of the prayer for rain.  When gentiles are included, they are defined as a bunch of individuals and therefore the Mishna uses the word נידונין, plural language.  In the first cases of the mishna, when it is only the avodah of Klal Yisrael, then it uses a singular language for Klal Yisrael comes together as one group.  The Rogatchover understands the din of tzibbur, comprised of various individuals is only applicable to Klal Yisroel, not to gentiles.  It is שבעים נפש, one נפש of the body of the nation only for בני יעקב.
The Briskor Rav understands the same way and says it as peshat in a possuk in Vaeschanan.

The Gemorah in Berachos (8a) says אמר הקב"ה: כל העוסק בתורה ובגמילות חסדים, ומתפלל עם הציבור - מעלה אני עליו כאילו פדאני לי ולבני מבין אומות העולם.  Why is praying with the tzibbur equated with redemption?  The Maharal (Netzach Ch. 10) explains that when the Mikdash is standing, it unites as all.  It is what defines us as one unit of Klal Yisroel, one tzibbur.  In golus, there is nothing to define us as a tzibbur.  Even if an entire body of Jews gather together, they are separate individuals that happen to be under the same roof.  What defines Klal Yisroel as a tzibbur?  Praying together before Hashem.  The state of being in front of G-d unites us together as a tzibbur.