They are they who...are sealed by the Holy Spirit of promise, which the Father sheds forth upon all those who are just and true.
They are they who are the church of the Firstborn.
These are they who have come to an innumerable company of angels, to the general assembly and church of Enoch, and of the Firstborn. - D&C 76:51-54 & 67

Q. What are we to understand by sealing the one hundred and forty-four thousand, out of all the tribes of Israel--twelve thousand out of every tribe?
A. We are to understand that those who are sealed are high priests, ordained unto the holy order of God, to administer the everlasting gospel; for they are they who are ordained out of every nation, kindred, tongue, and people, by the angels to whom is given power over the nations of the earth, to bring as many as will come to the church of the Firstborn. - D&C 77:11