Ok finally I resolve this issue and get invited friends ids.. :)
Just make on change in your .js file where you put your facebook connect javascript code. Use Get or Request Method instead of Post Method in fb:request-form tag.
var fbhtml = "<fb:serverfbml width="\""" width_of_invitation_div="" xmlns:fb="#unknown"><script type=\"text/fbml\"><fb:fbml><fb:request-form action=" + window.location + " method="REQUEST" invite="true" type="" + type_of_fb_request_form + "" hold=" /><br mode="></fb:request-form></fb:fbml></script></fb:serverfbml>"
After invitation send to users selected friends you get ids[] in url.
you can get these ids through query string and split ids by comma(,) and store in array.
Enjoy FBConnect.