concatenate cell values based on row number

Hi -

I have a huge list of cities and the users they are served by. The format looks lke this:

column A = row # (1, 2, 3, etc.), column B = username, column C = City

I have column B sorted by username ascending. It looks like this:

1 | Doug | New York
2 | Doug | Boston
3 | Doug | Cambridge
4 | Bob | New York

What I did was put this formula in column 4:
=IF(B3<>B2, A3, E2)

so that it now looks like this:

1 | Doug | New York | 1
2 | Doug | Boston | 1
3 | Doug | Cambridge | 1
4 | Bob | New York | 4

so basically it has the row number of the first occurrence of that username. The reason I want this is I need to concatenate all of the cities that each username serves, and I figured I could do this by doing something like:

if(D4 <> D3, concatenate(D[Value of D3]:D[Value of D4 - 1]), "")

But I have no idea how to reference the value of a cell and use it as another cell's row number. Is this possible? And can anyone think of a better way to do this?

Use INDIRECT:

if(D4 <> D3, concatenate(INDIRECT("D" & D3 & ":D" & D4 - 1)), "")