String aggregation in SSRS 2005

The usual way to do aggregate concatenation in SSRS is with custom code. See here for an example:

http://blogs.msdn.com/suryaj/archive/2007/08/11/string-aggregation.aspx

Here's the custom code in basic form:

Private CurrGroupBy As String = String.Empty
Private ConcatVal As String = String.Empty
Public Function AggConcat(GroupBy as String, ElementVal as String) as String
    If CurrGroupBy = GroupBy Then
        ConcatVal = ConcatVal & ", " & ElementVal 
    Else
        CurrGroupBy = GroupBy 
        ConcatVal = ElementVal 
    End If
    Return ConcatVal 
End Function

Followed by this expression at the grouping level you want to display:

=RunningValue(
     Code.AggConcat(
         Fields!YourFieldToGroupBy.Value
       , Fields!YourFieldToConcat.Value
       )
   , Last
   , "YourGroupName" 
   )

"YourGroupName" is typically "table1_Group1", if it is the first table and the first group you have created in the report, and if you didn't specify a different name.