This questions has multiple parts:
1. Are the 144,000 a special group different from the great multitude?
2. Or are both the 144,000 and the great multitude the same group?
3. If they are different are they both sealed or only the 144,000?

Nearest the throne are those who were once zealous in the cause of Satan, but who, plucked as brands from the burning, have followed their Saviour with deep, intense devotion. Next are those who perfected Christian characters in the midst of falsehood and infidelity, those who honored the law of God when the Christian world declared it void, and the millions, of all ages, who were martyred for their faith. And beyond is the “great multitude, which no man could number, of all nations, and kindreds, and people, and tongues, ... before the throne, and before the Lamb, clothed with white robes, and palms in their hands.” Revelation 7:9. Their warfare is ended, their victory won. They have run the race and reached the prize. The palm branch in their hands is a symbol of their triumph, the white robe an emblem of the spotless righteousness of Christ which now is theirs. {DD 54.3}