Situation: user A send invitation to my fb app to his friends X, Y and Z. Users X and Y has accepted invitations, but user Z not.
How can I track haw many users accepted invitations? I want to take some privileges for user A based on accepted invitations count.
Is there any method to track sender_id when invitation accepted?
Don't bother, it's not allowed by the policy.
Per section V.1.
You must not incentivize users to grant additional permissions or use Application Integration Points.
And per the documentation about Application Integration Points
By "Application Integration Point" we mean Application Info Section, Application tab, Feed, requests (including invites), Publisher, inbox attachments, Chat, Bookmarks, or any other feature of a user profile or Facebook communication channel in which or through which an application can provide, display, or deliver content directed at, on behalf of, or by permission of a user.
Actually, there is no need to additional permissions.
Using the next value, you can store the referring user id and referred user.
It's simply explained here: http://fbcookbook.ofhas.in/tag/tracking-invitation/