Concatenate Grouped Fields in SSRS

hello, need help here.

my table:

Company         |        Invoice

Company1       |         Inv1

Company1       |         Inv2

Company1       |         Inv3

Company2       |         Inv1

Company2       |         Inv2

------------------

my desired output:

Company         |        Invoice

Company1       |         Inv1 , Inv2 , Inv3

Company2       |         Inv1, Inv2

--------------------

i tried:

=Join(LookupSet(Fields!Company.Value
    , Fields!Company.Value
    , Fields!Invoice.Value
    , "DataSet1"), ",")

error:  Error 1 : LookupSet is invalid. InvalidIdentifier 

tried with Grouped Company, and Not-Grouped Company.

didn't work. how can I manage to do this? thanks.