Sliding effect inside a form application

Question

Hello

I have seen this article http://www.c-sharpcorner.com/UploadFile/kirtan007/761/ which shows how to make a slide effect,I want to know how can I make a slide effect inside a form,I mean I have a radio button and when I check it I that want two other buttons appears inside the mainform. 
I tried to put the radio button in a groupbox and i changed the code in the article into:

int WidthOfMain = Application.OpenForms["groupBox1"].Width;
int HeightofMain = Application.OpenForms["groupBox1"].Height;
int LocationMainX = Application.OpenForms["groupBox1"].Location.X;
int locationMainy = Application.OpenForms["groupBox1"].Location.Y;


But i get an exception,Can you help me please? Can you give me an idea ?
Thank you in advance 
Good day