Have you attempted anything or do you have any code to show us? It sounds like you're just asking for the code especially as it is custom. You could try getting a list of users friends, and have a checkbox by each one, then you can access the array of who they selected. Then look at publish_actions permission - though I don't think it would be the best performance wide to loop through each friend and make a graph post to them on behalf of the user. I know you want a custom solution but to be honest your best bet maybe to use the native friend selector and the js-sdk
https://developers.facebook.com/docs/reference/dialogs/requests/
Then use the callback and some ajax to save the information you require. Look at the requests example here as well
https://developers.facebook.com/tools/console/