In Psa. 100:1, the psalmist commands, "All the earth, shout to Yahveh!" Hence, the subject is "all the earth" (כָּל הָאָרֶץ) (cp. Psa. 33:8, 66:4, 96:1, 96:9, etc.).
To the same subject, "all the earth," the psalmist commands (Psa. 100:2), "Serve Yahveh with gladness!" (עִבְדוּ אֶת יַהְוֶה בְּשִׂמְחָה) and "Come in His presence with exultation!" (בֹּאוּ לְפָנָיו בִּרְנָנָה).
Then, to the same subject, "all the earth," the psalmist commands (Psa. 110:3), "Know that Yahveh is God! He made us, and we are His, His people, even the sheep of His pasture" (דְּעוּ כִּי יַהְוֶה הוּא אֱלֹהִים הוּא עָשָׂנוּ וְלוֹ אֲנַחְנוּ עַמּוֹ וְצֹאן מַרְעִיתוֹ).
Note: There is a Ktiv reading of ולא ("and not"), and Kri reading of ולו (perhaps, "and [we are] his").
Hence, Yahveh's people, even the sheep of his pasture (עַמּוֹ וְצֹאן מַרְעִיתוֹ), must be all the earth, whom the psalmist addresses in v. 1.
Keil and Delitzsch wrote,1
כּל־הארץ are all lands, or rather all men belonging to the earth's population.
The Targum of Psalms 100:1 interprets the Hebrew "כָּל הָאָרֶץ" into Aramaic as "כל יתבי ארעא," that is, "all the inhabitants of the earth."
1 Delitzsch, Frank; Keil, Karl Fredreich. Commentary on the Old Testament.