The Mishnah says that in the times of the Beis HaMikdash, they would send out messengers in Iyar for Pesach Sheni. However, Pesach Sheni is only relevant for people who are in Yerushalim on the 14th of Iyar (in other words, anyone obligated in Pesach Sheni must be present in Yerushalayim on the 14th to offer their offering). Therefore, I would think that people generally would not have time to wait for the messengers to arrive. They would simply show up in Yerushalayim on the day that they believe is either the 13th or the 14th. If it's the 14th, fine. If not, they simply wait and offer the offering the next day. I suppose the simple answer is that messengers would only travel out a week's distance, since anyone beyond that point would not be able to get to the Beis HaMikdash in time anyway, but it seems strange to send out messengers to a group of people who are coming to Yerushalyim anyway. I guess you could ask the shailah on any of the Shalosh Regalim, except for those, people beyond a certain distance are patur from bringing the offering, though many did. But does someone who missed Pesach have a higher obligation to come to the Beis HaMikdash for Pesach Sheni?
Avrohom Adler, usa
This is an excellent question I heard years ago. So far I have yet to find someone who asks it.
Y. Landy
(a) Certainly the messengers would only help those who were near enough to Yerushalayim to be able to wait until the messengers arrived, and still be able to arrive on time to bring the Korban. Those who lived further away would have no choice but to leave their homes in enough time to be sure that they would arrive in time to bring the Korban. But for those people who were close enough the messengers would help them by enabling them to stay at home for longer, and not to have to come to Yerushalayim earlier than necessary if the month was Malei.
(b) If a person missed Pesach Rishon negligently (b'Meizid), and did not bring the Pesach on Pesach Sheni, even by mistake (b'Shogeg), he would be punished with Kares (Rambam Korban Pesach 5:2). Therefore a person who missed Pesach Rishon would have to take great care not to miss Pesach Sheni, and perhaps as an additional reminder, they sent messengers for Iyar.
Dov Freedman