There are many sources which mention Guadalupe from the 1550s onward (especially Spanish ones), but there are only a few sources (many of which are vague and/or disputed) which mention it from before that date. 1555-6 is the watershed, when a Spanish
ganadero (cattle rancher) was cured of his illness after praying to Our Lady of Guadalupe.
You might even see a hint of it in the nature of the original shrine at Tepeyac (as per the testimony of the Indians). The original chapel (a very small adobe house) was not a church properly speaking: there was no priest, nor even a resident staff (people from Cuautitlan regularly visited and maintained the shrine). It was a lay shrine or oratory built and maintained by the natives.
The mass conversions of the natives started in 1536 onwards. At that time, not even the friars knew what the exact cause was. At the time, Fray Toribio Motolinia (one of the
Twelve Apostles of Mexico) noted that inexplicably, Indians began to appear by the thousands to him, asking to be baptized. He was close with the natives, but even he did not know what caused them to convert. A disciple of Motolinia, Fray Gerónimo de Mendieta, suggested that the conversions might have been caused by a prominent native ruler (
tlatoque) converting to Christianity: the people were following suit. (This was a common pattern: when a ruler converts, so did the people under him. The same thing happened in Japan.)
However, none of the friars directly linked the mass conversions with Guadalupe. In fact, they treated the mass conversions with some suspicion: as I noted earlier, Fray Zumarraga began to use his inquisitorial powers to prosecute superstition and (relapse to) idolatry among the natives around the period 1536-1543.