Concatenating Cells Based On Common Value In Next Column?

I'm using excel 2008 for mac. I've got a spreadsheet with around 4000 rows. The data is organized like this:

|___________URL_________|______EMAIL______|
|Example Domain
|Example Domain
|Example Domain
|Example Domain
|Example Domain

What I'm trying to do is concatenate all of the URLs into one cell so that each email address has only 1 single row with many URLs associated with it. I tried using VLOOKUP(B1, $A$1:$B$4000, 1, FALSE) but this didn't work. I thought it was a long shot anyway. I also tried writing a bash script do parse a csv with this data and create a csv file with the URLs concatenated with no luck. I'm trying to get my data to look like this:

|___________URL_________|______EMAIL______|
|Example Domain
|Example Domain
|Example Domain
|Example Domain
|Example Domain

Where these are 2 adjacent cells.

EDIT: The format didn't turn out like I'd hoped. In the first example, each URL has an associated email address, each in its own row. In the second, it is just one row, with multiple urls in the first cell and an email address in the second cell.