How about this expression?
(From d In BuildDT.AsEnumerable
Group d By k=d("Id").toString.Trim Into grp = Group
Let nj = String.Join(" ",grp.Select(Function (n) n("name").toString.Trim))
Let ra = New Object(){k,nj}
Select r = DtOutput.Rows.Add(ra)).CopyToDataTable
DtOutput → BuildDT.Clone
Regards
Gokul