61 And now I, Alma, do command you in the language of him who hath commanded me, that ye observe to do the words which I have spoken unto you.
62 I speak by way of command unto you that belong to the church; and unto those who do not belong to the church I speak by way of invitation, saying: Come and be baptized unto repentance, that ye also may be partakers of the fruit of the tree of life.
I love how Alma emphasizes his authority to those who are members of the Church and then extends an invitation to those who are not to come and be baptized and join the church. 
As members of God’s church, we have Covenanted with God to follow Him and to sustain, uphold, and support the leaders of his Church. When they speak, we are obligated by virtue of our covenants to hearken to their words as a commandment of God. They have spiritual and ecclesiastical authority over us.

Those not yet members are not yet under such covenant. Yet, God’s prophet and the high priest of his Church speaks the will of God to all the world. He invites all to listen to the word of God. He invites all to be partakers of the blessing of being led by a living Prophet and continual revelation from the mouth of God. He invites all to prepare to be baptized and make and keep other sacred covenants. That invitation is one that goes out to all the world.