The reference to the "great crowd" in ch. 7 is just as much in heaven as the "great crowd" in ch. 19, regardless of whether they are identical. They are "before the throne of God (enòpion tou thronou theou) and serve him day and night in his temple (en tò naò autou)" (7:15). Those who are also "before the throne" are the seven spirits in 1:4, the seven lamps in 4:5, the sea of glass in 4:6, "all the angels" a few verses earlier in 7:11, the golden altar in 8:3, 9:13; these are heavenly objects or beings. The term enòpion also in this context implies proximity, "in front of"; cf. "The twenty-four elders will fall down before (enòpion) him who sits on the throne ... and will cast their crowns before the throne (enòpion tou thronou)" (4:10), the twenty-four elders are heavenly, seated on their own thrones surrounding God's throne (4:4, 11:16), and this involves a casting forth of crowns towards the throne. The "temple" (naos) is also explicitly designated as "in heaven" in 11:19, 14:17, 15:5.
The great crowd has "come out of the great tribulation". This means one thing: they are dead; they were martyred during the tribulation. This innumerable crowd completes the number of martyrs mentioned in 6:9-11: "I saw under the altar the souls of those who had been slain because of the word of God....Then each of them was given a white robe, and they were told to wait a little longer, until the number of their fellow servants and brothers who were to be killed as they had been was completed". The reference to the incomplete number of martyred saints foreshadows the innumerable great crowd coming out of the great tribulation in the next chapter. These are the dead in heaven (as the "altar" is in the heavenly temple), awaiting the completion of the tribulation. Notice too that they are given a white robe (an angelic garment), just as those of the "great crowd" in heaven in 7:13. These members of the "great crowd" have white robes because they have "made them white in the blood of the Lamb" (7:14). This again means that they are martyred, for the martyrs in 12:11 have overcome the Devil "by the blood of the Lamb and by the word of their testimony, for they did not love their lives when faced with death". Notice the past tense. They are already dead, like the martyred saints in ch. 6 and 7.