Re: The poem begins with an "I." To what extent does the poem refer to the individual and to the community?
Whitman refers to himself, yes, but his "I" often includes everyone else too. We tend to assume the speaker here is Whitman, which is sometimes a tricky assumption (though it seems safe in this instance). In the first group of lines, he says,
I celebrate myself, and sing myself,
And what I assume you shall assume,
For every atom belonging to me as good belongs to you.
The poem isn't just a celebration of Whitman, or of the speaker, but a celebration of each one of us. What he assumes about himself is what we, too, should assume about ourselves. Everything that makes him good and wonderful makes each of us good and wonderful as well.
The speaker invites us to join him, not just to listen to him, saying,
Stop this day and night with me and you shall possess the origin of all poems,
You shall possess the good of the earth and sun, (there are millions of suns left,)
You shall no longer take things at second or third hand, nor look through the eyes of the dead, nor feed on the spectres in books,
You shall not look through my eyes either, nor take things from me,
You shall listen to all sides and filter them from your self.
He doesn't want us to take his word for things but rather to see and discover them for ourselves.