The passage is found in the so-called Fourth Servant Song (Is 52:13-53:12). The overall structure can be outlined as having two oracle-like statements pronounced by YHWH (Is 52:13-15; 53: 12, or better: 53:11aβ-12) framing a collective account of the deeds of the Servant (Is 53: 1-11, or better: Is 53:1-11aα); for an in-depth analysis of the structure and poetic form of the text see J. Goldingay-D.Payne, Isaiah 40-55. Vol.2 (ICC), T&T Clark, London 2006, pp. 275-9. It is in this collective account that multiple times a I pers. plur. is used. P.D. Hanson, Isaia 40-66, Claudiana, Torino 2006 [Eng.ed. 1995; I just own the Italian edition, so I quote from that and re-translate in English], pp. 172-3 discusses the issues of the plural person:
«The description, in 53:1-11, is made in the first person plural, which raises the question of the identity of these “we” who contemplate the Servant, adding the astounding claim that, through his suffering, God has forgiven their sins and healed them. The reference to kings and nations in the preceding verses has suggested to some that it is the peoples of the earth who are speaking here, an idea not to be ruled out, in a horizon of thought in which the servant is depicted as actively carrying on God’s work […] Following the majority of Jewish and Christian commentators, however, it seems more accurate to recognize, in those describing the servant, the voice of the Jewish community.»
J. Blenkinsopp, Isaiah 40-55 (AB 19), Doubleday, New York, pp. 351-2 seems to agree on this point:
«The empathic language of 53:1-12 also renders it unlikely that the speaker represents the nations and their rulers mentioned in the Yahveh discourse. The eulogist is an individual, almost certainly a disciple, as noted earlier, and one who speaks on behalf of those who “revere Yahveh and obey the voice of his Servant” (50:10). The form of the discourse, which spans the life of the Servant from his early years to his death and burial, may have borrowed phrases from the repertoire of lament psalms but reads more like a panegyric pronounced over the catafalque. A purely formal comparison with the panegyric of Julius Caesar as reported by the historians Appian and Dio Cassius, which included the benefits that Caesar conferred on others and his sufferings and violent death, would not be out of place.»