How to request permission to Join.

Does anyone know how to add the contact form to the Join button on the home page ?    http://3d-story.com/test/

I want a person joining my group to request permission to join. So I would like a contact form to be seen here.

Also once the contact form is attached to the join button how do I edit the required fields in that contact form  ?

Thank you.

Can anyone help with this problem ?

I want someone to ask for permission to join the site before they can proceed to create a profile. Can anyone suggest how I can do that ?
So the Join button would actually be a contact form.

Any ideas how to achieve this ?