Revelation 5:5

5 And one of the elders saith unto me, Weep not: behold, the Lion of the tribe of Juda, the Root of David, hath prevailed to open the book, and to loose the seven seals thereof.

One of the elders tells John not to weep. This verifies the assumption that neither John nor the apostles are part of the twenty-four elders. If the one talking to John knew about these things as an elder, then John as one of the twenty-four elders would also have known these things. In fact, it is likely that he would have known all the elders by name.

The celestial elder tells John that the Lion (King) from the tribe of Judah (Jesus was from Judah) " has the power"  to open the book (the words " had prevailed"  in the King James seems to give a different meaning in today's English than the author intended).

Revelation 5:5  And one of the rulers said to me, Do not be sad: see, the Lion of the tribe of Judah, the Root of David, has overcome, and  has power  to undo the book and its seven stamps. BBE

Revelation 5:4 says no man was worthy but this verse says there is one worthy. This sounds like a contradiction but it is not, because  Jesus was not born from the seed of a man. He was born of God but He also qualifies to be a kinsman redeemer because His human blood lines came from His mother. He was born a man from the seed of the woman.

Genesis 3:15 And I will put enmity between thee and the woman, and between thy seed and  her seed; it shall bruise thy head, and thou shalt bruise his heel.

He is the root of David because His royal blood line is from David. The fleshly genealogy from Eve to David to Mary is given in the gospel of Luke while the genealogy that gives Jesus the authority to be king of Israel is given in the genealogy of Matthew. Jesus in the end will also fulfill the role of  the avenger of blood  (the person in the Old Testament that legally sought out and killed the murderer of his brethren). Jesus will put down all authority of the angel who through his deception brought sin and death into the world and He will cast him and his followers into the place of final death (the Lake of Fire).