Rav Yosef says (as Rashi explains) that we decide whether most of the people are Temei'im based exclusively on the people that we find in the Azarah.
I don't understand - how do we let the Temei'im in in the first place, if we are not yet sure that the Pesach will be brought b'Tum'ah (since we have not yet found Temei'im in the Azarah)!
Pesach Feldman, Yerushalayim, Israel
The Gemara does not literally mean the people who are presently standing in the Azarah, but rather the people who are about to enter the Azarah (as opposed to including the people who are Menuyim on the Korban). This is clearly stated by the Yerushalmi (7:6) where Rebbi Yosi adds "Ad she'Hein miba'Chutz Misharin Es Aztman" -- "When they are still outside they take measure of themselves (regarding how many are Tamei)."
I think when Rashi says that this excludes those who are "ba'Chutz," he is also talking about the Menuyim who are not planning on entering the Azarah, not those who already entered. This is also the way the RAMBAM quotes the Halachah (Hilchos Korban Pesach 7:6).