You can use the normal Request Dialog - link
You just have to provide the to
parameter with the request, eg:
to=FRIEND1,FRIEND2,FRIEND3
Here's a direct URL, you can test first:
https://www.facebook.com/dialog/apprequests? app_id=YOUR_APP_ID& message=MyMessage& redirect_uri=LINK_TO_YOUR_APP&to=FRIEND1,FRIEND2,FRIEND3