D&C 13

Being under the direction of Peter, James, and John, the prophet John the Baptist returned to confer the lesser or the Aaronic Priesthood upon Joseph Smith and Oliver Cowdery saying -

"UPON you my fellow servants, in the name of Messiah I confer the Priesthood of Aaron, which holds the keys of the ministering of angels, and of the gospel of repentance, and of baptism by immersion for the remission of sins; and this shall never be taken again from the earth, until the sons of Levi do offer again an offering unto the Lord in righteousness."  (Doctrine and Covenants | Section 13:1)

With the words "keys" and muliple "and" suggests to me that the ministering of angels, the gospel of repentance, and baptism are interrelated.  First we must understand, in my mind, that ministering angels are individuals are messengers sent from God.  It is my opinion that an angel can then be a spirit, a earthly human being, and also a resurrected being.  Any one of these could be messengers sent from God.  I would even consider the Holy Ghost could be considered an angel by this definition.   Moroni asks -

"...my beloved brethren, have miracles ceased?..."  (Book of Mormon | Moroni 7:29)

And then he teaches -

"...Nay; neither have angels ceased to minister unto the children of men.
30  For behold, they are subject unto him, to minister according to the word of his command, showing themselves unto them of strong faith and a firm mind in every form of godliness.
31 And the office of their ministry is to call men unto repentance, and to fulfil and to do the work of the covenants of the Father, which he hath made unto the children of men, to prepare the way among the children of men, by declaring the word of Christ unto the chosen vessels of the Lord, that they may bear testimony of him.
32  And by so doing, the Lord God prepareth the way that the residue of men may have faith in Christ, that the Holy Ghost may have place in their hearts, according to the power thereof; and after this manner bringeth to pass the Father, the covenants which he hath made unto the children of men."  (Book of Mormon | Moroni 7:29 - 32)

I note that angels show "themselves unto them of strong faith and a firm mind in every form of godliness."  Which would suggest to me that they show themselves under those that have repented and been forgiven and worthy of baptism for remission of sins...even having been baptized.  Very similar to the requirements to receive the Holy Ghost.  Thus with the administration of the authority and power of the Aaronic Priesthood, in repentance, baptism, and the sacrament is the key or allows the saints to be blessed with the ministering of angels. 

Elder Dallin H. Oaks taught -

“Nephi described three manifestations of the ministering of angels when he reminded his rebellious brothers that (1) they had ‘seen an angel,’ (2) they had ‘heard his voice from time to time,’ and (3) also that an angel had ‘spoken unto [them] in a still small voice’ though they were ‘past feeling’ and ‘could not feel his words’ (1 Ne. 17:45). The scriptures contain many other statements that angels are sent to teach the gospel and bring men to Christ (see Heb. 1:14; Alma 39:19; Moro. 7:25, 29, 31–32; D&C 20:35). Most angelic communications are felt or heard rather than seen. “How does the Aaronic Priesthood hold the key to the ministering of angels? The answer is the same as for the Spirit of the Lord. “In general, the blessings of spiritual companionship and communication are only available to those who are clean. As explained earlier, through the Aaronic Priesthood ordinances of baptism and the sacrament, we are cleansed of our sins and promised that if we keep our covenants we will always have His Spirit to be with us. I believe that promise not only refers to the Holy Ghost but also to the ministering of angels, for ‘angels speak by the power of the Holy Ghost; wherefore, they speak the words of Christ’ (2 Ne. 32:3). So it is that those who hold the Aaronic Priesthood open the door for all Church members who worthily partake of the sacrament to enjoy the companionship of the Spirit of the Lord and the ministering of angels.” (Dallin H. Oaks, “The Aaronic Priesthood and the Sacrament,” Ensign, Nov. 1998, 39.)