How we cite our quotes:
For we were nursed upon the self-same hill,
Fed the same flock; by fountain, shade, and rill (23-24)
Let's follow this poem's metaphor all the way through, shall we? If the speaker is Milton, and his shepherd buddy is Milton's fellow poet, Edward King, then the fact that they were "nursed upon the self-same hill" might mean that Milton thinks he and King are descended from the same poetic ancestors, that they were raised on the same verse (like, say, pastoral poetry). If this is the case, the speaker is picking up where Lycidas left off, or, rather, Lycidas' death makes room for the speaker to occupy the poetic hill all alone.
Together both, ere the high lawns appeared
Under the opening eye-lids of the morn,
We drove afield (25-27)
These two really had it made in the good old days, huh? The fact that they both were shepherds again suggests that Lycidas was a fellow poet, a member of the same distinguished tradition. The speaker here is more interested in the fact that the two were friends, though, than in what they did. Note how the phrase "together both" comes first, and how the verb ("drove afield") comes almost two lines later.
There entertain him all the saints above,
In solemn troops, and sweet societies
That sing, and singing in their glory move,
And wipe the tears forever from his eyes (178-181)
In the second half of the poem, Lycidas exchanges one group of friends, his fellow shepherds, for another – the "sweet societies" of angels in heaven. If we read this metaphorically, we might take this to mean that he has left his fellow Cambridge students (the shepherds) for the company of angels. Thinking of Lycidas' or King's new friend up in heaven just might help ease the pain of his comrades back on earth.