Depending on the context, the phrase "Kingdom of God" can have a few different simplified meanings:
1: The authority of God manifesting itself through miracles, love, or judgment.
2: The authority of God in people's lives.
3: The Lord Jesus, the 12 Apostles, and others who will sit on Thrones and wear Crowns in the New Heavens and the New Earth.
4: The 1,000 Year Reign of Christ on Earth (if that is literal instead of symbolic).
5: (In the Old Testament:) The nation of Israel under godly kings such as David or Josiah.
There are a few passages of Scripture I can think of which could support the idea that the Kingdom is made up of those who rule in Heaven, as opposed to those who live on earth. These are Matthew 11:11-13, John 3:13, 31, Acts 2:34. Delving into those Scriptures will be the subject of a future post.
As always, the immediate context of a passage, and the context of that particular book as a whole, helps determine the meaning of the phrase.