Alma has preached to the people of Noah in their homes, and in secret. While this was apparently essential, it would also be very unsatisfactory for those who believed him. Believing would inevitably stir a desire to participate in a community of believers, to share their common understandings. The gathering of larger numbers of believers in the politically charged atmosphere of Lehi-Nephi would likely be nearly suicidal. In order to be able to openly live their new understanding of their religion, the believers would have to leave their established community and begin a new one. Note that those who are going to the land of Mormon are those who already believe, not those who are interested and simply want to learn more. It is most likely that the location of the community was also kept secret, and was known only to the faithful, lest a spy give them over to the king's men.
Just as Joseph's earliest converts found that gathering to a new community gave them the opportunity to openly share their religion with others of the same beliefs, so did this community of Alma's believers. They have removed themselves from their homes, and gone to a relative wilderness, not to seek riches, but to seek the kingdom of God. They left home and perhaps family for a place that might be beautiful, but was at least somewhat dangerous (see the comment on the wild beasts, verse 4). The process of the gathering takes multiple days. Some of that time would be in travel, some was taken up in the preparations for that travel. Eventually four hundred and fifty people join Alma (see verse 35). We may presume that this count is likely to be of adults only, as children are not mentioned, and converted families would certainly bring their families. If the count is four hundred and fifty adults, there would probably be somewhat more than half of that number as households, which would create a hamlet. Of course if the count follows the general Old World custom of counting adult males only (a distinct possibility since the Book of Mormon is so studiously quiet about women) then we have a much more sizeable village. Either number would have created a noticeable departure from Lehi-Nephi, and would certainly have generated official governmental concern, as is indicated in the history we will see unfold.
Our quick introduction to the essential difference in what Alma taught from that which they might have learned in Lehi-Nephi is found in the final sentence of verse 7. They learn "repentance, and redemption, and faith on the Lord." These phrases are sufficiently common for modern Christians that it is easy to gloss over their significance for Alma's community. What Alma preaches is the atoning Messiah, precisely the message that Abinadi delivered, but which was not believed in Abinadi's ministry. The conversion of Alma by Abinadi succeeded in doing what Abinadi had been unable to do, to convert some of the people of Noah to the true understanding of the connection between the Law of Moses and the coming atoning Messiah, Christ the Lord.
It is equally probable that modern LDS miss the newness of Alma's preaching. With Alma we see the first clear evidences of the modern church in the Book of Mormon. With Alma we will see an organization of the community of believers that was not only innovative, but revolutionary. What begins in this small group will later become the pattern for all Nephite religious practices. While Alma begins with the message of Abinadi that echoed the Christological message of the Nephi, he adds to this timeless message his own revelatory expansion and development. As we see the story of Alma develop, we will see Alma as the most important religious and political innovator in Nephite history. See Daniel C. Peterson's "Priesthood in Mosiah." The Book of Mormon: Mosiah, Salvation Only Through Christ. Religious Studies Center, BYU, 1991, pp. 187-210 for an important discussion of priesthood and church in this period of the Book of Mormon.