Who are "we"? That's a great question and we have a few possible answers too. The "we" that we get in Dunbar's poem is primarily directed towards black American people as a whole. However, we can also read it in a more general sense that's focused on all people. If we read it both ways, we end up with a perspective that's universal and specific all at the same time. But since that "we" is so ambiguous, it also serves a literary purpose too in suggesting that maybe the poem also wears a mask that hides its specific subject.
Title: Having "we" in the title really adds to the literary purpose of suggesting that the entire poem wears an ambiguous mask that's hiding the truth. The speaker isn't omitting specifics because he forgot about them. He's clearly doing it with a purpose.
Line 1: It's in the title and in the refrain. It also invites the reader to include himself in that "we." All in all we can apply the idea of wearing masks to any person we choose.
Lines 3: First we get more of the sense that the speaker is speaking to a universal audience with the phrase "human guile." The "we" shares this experience of "human guile," so it's not limited to just the black American struggle.
Line 4: The "we" is maybe getting a little more specific, since the speaker talks about "torn and bleeding hearts" which suggests that these wounds are fresh. And historically speaking, they were very fresh.
Lines 9, 15: In the refrain we're allowed to be as universal or specific as we want. The phrase "we wear the mask" stands alone in these lines so the reader can draw as many connections as he chooses.
Lines 10, 12: Just like that "smile," the "we" in these lines functions in a similarly dualistic sort of way. On one side the "we" is "smiling" and "singing," while on the other the "we" is "crying." In the end, that "we" is torn and not being honest.