It is unclear what action was performed at this time. The twelve had already been preaching, and now after prayer they minister to the people. The combination of ministering and the exact repetition of the worlds that Jesus had spoken indicates an ordinance more than a teaching occasion. Our hint here is that “they desired that the Holy Ghost should be given unto them.” In this context, it would appear that the particular ministration was giving the gift of the Holy Ghost, for that is also what appears to have been given to the twelve on the previous day. (3 Nephi 18:36-37).
This is only slightly problematic for modern audiences, particularly because the very next event will be mass baptisms. It would appear that the order of things is wrong, as we understand that we are baptized first, and then given the Gift of the Holy Ghost.
It is most probable that the people who were in Bountiful were already believing members of the church, and that they had gathered together in Bountiful on the basis of that commonality. They would already have been baptized, and therefore able to receive the next ordinance. Of course this explanation raises yet another question, for we also understand in modern practice that one need only be baptized once. However, re-baptism was not unusual in the early modern church.
“Elder Andrus left Salt Lake City more than two months before Elder Snow and on 30 May arrived in St. Louis where he was met by Orson Pratt and Horace S. Eldridge. While waiting for Erastus Snow he kept busy. In one of his letters to the Deseret News he reported:
I began to feel after the Saints and found many disaffected and the Holy Spirit came upon me, when I thought of the best plan to save the most: and I counseled them to renew their covenants by rebaptism, and by making new records as the old were imperfect. I also opened the door to those who had been cut off, only forbidding such as were forbidden by all laws this side of the mountains. The result is, the Saints are rejoicing and bear testimony that they have never felt better in their lives; and about twenty-five more have been baptized, some of whom had been cut off… .” (The Saints and St. Louis, 1831-1857: an Oasis of Tolerance and Security by Stanley B. Kimball Fn, BYU Studies, vol. 13 (1972-1973), Number 4 - Summer 1973 512.)
“The Church-wide home missionaries created the style and traditional procedures which were used in subsequent missionary work to the Saints, and which persisted after the cause of home missionary work had withered. In 1855, a widespread vigorous missionary work was conducted both by regular officers of the Church and special missionaries. The need for reform was preached from the pulpit and taught in the homes. Repentance and rebaptism were the results of this concerted effort to bring the Mormons to a more spiritual style of living. James S. Brown described his labors as a missionary during the reformation by indicating that from 1856 to 1859 he baptized or rebaptized 400 persons and visited with the catechism from house to house.” (Missionaries To the Saints by A. Glen Humpherys , BYU Studies, vol. 17 (1976-1977), Number 1 - Autumn 1976 76.)
“For many years [in the nineteenth century] it had been common for members to rededicate themselves to building up the Kingdom through rebaptism. This practice was not considered essential to salvation but was a symbol of rededication. On other occasions the Saints were rebaptized as a symbolic gesture related to blessings for their health, entry into the United Order, preparation for marriage and even for going to the temple if they had not been there for some time.” (The Practice of Rebaptism at Nauvoo by D. Michael Quinn Fn, BYU Studies, vol. 18 (1977-1978), Number 2 - Winter 1978 226.)
While rebaptism is no longer practiced, there is no reason to assume that it could not have been practiced by the Nephites. In their case, there was an additional reason for being baptized again. Although their baptism was for remission of sins, it was a baptism where the symbolic force was that of water for cleansing. After the resurrection of the Savior, the Christian baptism took on an additional symbolic context of the death and resurrection. Thus the Nephites were renewing covenants, and transforming their religious practice from the pre-Christian symbolism to the post-Christian symbolism. It was fitting that they should renew their covenants by renewing a baptism that was now even more intimately connected to the Atonement and the Savior.