Wondering if there is a simple formula to concatenate row 1 values based on
the criteria of "df" for each date. This formula could be placed in another
column. Realize that eventually there will be upto 70 people and 365 Days.

A B C E
Bill Joe Frank
1-Jan-10 df df
2-Jan-10 df df df
3-Jan-10 df
4-Jan-10 df
5-Jan-10 df df
6-Jan-10 df

The desired outcome would be something like this:
1-Jan-10 Bill, Frank
2-Jan-10 Bill, Joe, Frank
3-Jan-10 Joe
4-Jan-10 Bill
5-Jan-10 Joe, Frank
6-Jan-10 Frank