Get a list of selected friends without sending messages

My online game site uses Facebook as one of authentication backends.

I am using fb:request-form to allow a user to invite friends to the site. But if a friend has already joined, instead of a Facebook notification I want to send an in-game 开发者_运维技巧message.

Is it possible to get a list of selected userids without sending messages?

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Just check if user exists in your game already. If yes, send him message instead of invitation.

Use powers of AJAX