In Mosiah 18:35 we learned that there were about four hundred and fifty people who went with Alma. That is not a very large number. It could sustain an agricultural community, but it is somewhat surprising that they would seek a king with so few people. There was clearly a strong cultural imperative for kings, something that explained the reason Nephi’s people also desired a king. What they apparently had was a theocracy, where Alma as the high priest was also the de facto head of the government.
As their high priest, he assumed the rights and authority to give others the authority to teach and perform ordinances. In Israel, it would have been the right of a king, but in Alma’s community it rested with Alma as the combined religious and secular leader. Of course, divine recognition of that authority was also required, but Alma clearly had received that, even though it is not explicitly described.
Mormon reiterates the rule of just men by noting their actions, which was to care for others. That was part of the definition of a just ruler from verse 15 and it is shown in its fulfilment here.