Who stand in Beis Hashem?
Radak (2-3): The Kohanim. This refers to those who will stand in the Bayis that Shlomo will build, or in the Bayis that Melech ha'Mashi'ach will build.
Malbim #1: It is the Kohanim who are close to Hashem, and enter Beis Hashem.
Malbim #2: This is those who grasp the King Himself - they know His secrets and hidden matters, e.g. Ma'ase Bereishis and Ma'ase Merkavah.
Who stand in "Chatzros Beis Elokeinu"?
Radak: Yisrael; they would pray in the Azaros, for they may not enter the Heichal.
Malbim #1: It is the Leviyim and Zera Yisrael, who grasp Hashem via His known name.
Malbim #2: It is those who stand in the Chatzeros outside; they know Him only from afar.
