Pertaining to the OP, 24 is the number of priests (c.f. King David diving the priests into 24 groups).
The two main theories I heard (in which I'm not pushing either one on people) is that 1) these are saints in heaven who will come back with Christ if you believe in the pre-trib rapture, which means the first resurrection also occurred except for some "Tribulation saints" who are converted afterwards also join them at the Second Coming. The verse they use is "you have made us kings and priests".... If the translation is correct, then the pre-Tribbers have a point except if these could be those few resurrected with Christ and then ascended, just part of the church, the first of the saints but not all of them which is an alternate, similar theory.
2) These are the angelic priests in heaven. When they sing the song, the translation (some say) is "you have made them kings and priests" and are singing for and on behalf of the saints presently undergoing persecution on this earth (take note that the prayers of the saints are compared to the altar of incense going up to heaven as well). If this translation is true, then I can see how this could be a heavenly choir.
There was a time when I was more adamant in what I believed, and thought it was all figured out, but then again I can see other viewpoints have validity as well. Whether talking about rapture/second coming, BOTH seem to be connected with the feast of Trumpets taking place at that time. Now some separate rapture/second coming as separate events, which means the Trumpets feast has to occur twice and have two fulfilments in the future. I lean to just one, simply because I'm big into time, and what the feast represents, so my position was not influenced by any one group over another, but rather just the idea of (for want of a better word) God's "scheduling".