RadGridView のデータソースとして使用する DataTable を Form1 内で Public として宣言し、Form2 内で RadGridView の代わりに DataTable に行を追加します。
Form1 の内部
Public DataTable1 As New DataTable
Private Sub Form1_Load(sender As System.Object, e As System.EventArgs) Handles MyBase.Load
DataTable1.Columns.Add("data")
DataTable1.Rows.Add("Test row from Form A")
RadGridView1.DataSource = DataTable1
End Sub
Private Sub Button1_Click(sender As System.Object, e As System.EventArgs) Handles Button1.Click
Dim Form2 As New Form2
Form2.ShowDialog()
Form2.Dispose()
End Sub
Form2 の内部
Private Sub Button1_Click(sender As System.Object, e As System.EventArgs) Handles Button1.Click
Form1.DataTable1.Rows.Add("Test row from Form B")
End Sub