The nature of Alma's community receives a little more explanation in this verse. What Noah sees is a "movement among the people." We must first understand what Noah saw. It might be that he would describe Alma's new church as a "movement," but his actions suggest that what he saw was a physical movement, or a number of people who were leaving the city to meet with Alma. It is this movement of people that he notices, and sends spies to trace. Thus when they had met "they were discovered unto the king," meaning that their place of hiding was uncovered because the spies had followed them.
This movement of people in the city to Alma in the land of Mormon raises a question about the nature of his community. Was this a community separate in physical location, or only of spiritual belief and intra-community interactions? Our clarification of this point comes much later in the text, in Mosiah 23:1:
"Now Alma, having been warned of the Lord that the armies of king Noah would come upon them, and having made it known to his people, therefore they gathered together their flocks, and took of their grain, and departed into the wilderness before the armies of king Noah."
This verse is very clear in its representation of a separate community. We must therefore return to the "movement" of people. It would appear from this evidence that Alma's preaching created did not create an irrevocable separation from Lehi-Nephi. There were those who stayed in Mormon, and those who returned to Lehi-Nephi while remaining spiritually connected to Alma. It is these "commuters" that were seen leaving the city, and who were followed to the meeting place.
As was noted earlier, the expansion of adherents from two hundred to four hundred and fifty in a relatively short time requires missionary work in Lehi-Nephi. In this "movement" of people we see the mechanism of the missionary work. Whether overtly or covertly, those who had accepted their Redeemer returned to Lehi-Nephi and found others would were willing to accept the atoning Messiah. Those accompanied them to Mormon, and were baptized there into the church of God.