The Mishnah says:
The entire community, all of Israel, became tamei with the tumah of a corpse before Pesach.
The majority of the community became tamei with the tumah of a corpse.
The community is tahor, and the majority of Israel is tahor, but the kohanim are tamei.
In each of these situations there is tumah, and the offering is brought in a state of tumah. Those who are tamei and those who are tahor enter the Azarah - the courtyard - together, "lefi she'ein mechalkin korban tzibbur" - because a communal offering is not divided, meaning that the offering of the many is not split up.
And this applies even to individuals: since the majority of the community is tamei, even the individual who is tahor brings his offering in a state of tumah. And of course, as we mentioned, this ruling applies only to the tumah of a corpse, and not to other types of tumah.
And what is the law when only a minority of the community is tamei?
In this case we return to the usual order:
Those who are tahor - who are the majority, bring the Pesach offering at its proper time, on the fourteenth of Nisan.
Those who are tamei - like anyone who became tamei and cannot bring the Pesach offering because of his tumah, bring it on Pesach Sheni, on the fourteenth of the month of Iyar.
Pesach Sheni is the time of completion that Hashem established for someone who did not manage to bring the Pesach offering at its proper time in the month of Nisan, and he is able to bring it a month later.
In summary: when the community or its majority is tamei with the tumah of a corpse, and likewise when the kohanim are tamei and the community is tahor, the Pesach offering is brought in a state of tumah, and those who are tamei and those who are tahor enter the Azarah together, since a communal offering is not divided; but when only a minority of the community is tamei, those who are tahor bring the offering on the fourteenth of Nisan, and those who are tamei complete their obligation on Pesach Sheni, on the fourteenth of Iyar.