Styling of registration form

If you want to just add additional HTML after the form, you could use the wpmem_register_form filter to append to the form’s HTML. Something like this:

add_filter( 'wpmem_register_form', 'already_registered_link' );
function already_registered_link( $form ) {
     $new_html = '<a href="https://mysite.com/my-login/">Already Registered</a>';
     $form = $form . $new_html;
     return $form;
}

Hi Chad,
thanks for your support. However it add a link below the form.
I tryed to adapt your code to add a button inside the form:

add_filter( 'wpmem_register_form_buttons', 'already_registered_link' );
function already_registered_link( $buttons ) {
     $new_html = '<div class="button_div">Already registered?&nbsp;<a href="https://mysite.com/my-login/"><input class="buttons" type="button" value="login"></a></div>';
     $buttons = $buttons . $new_html;
     return $buttons;
	 }

It’ ok on FF and Chrome but I’m not able to make it work on IE. Nothing happens clicking the button.
Could you help me again in this matter?
It would be nice to have a button like that by default or as an option in the registration form.

Ok…I finally kept the link and modified the CSS to make it look like a button.
I’m a beginner so, if there is a more clever way to do the job, please let me know.