User added\deleted event in sharepoint

How can i know when a user is added (or deleted) to a sharepoint site or to a group in a sharepoint site?

I tried to attach an SPItemEventReceiver on the user information list, but the main problem is that i can't know when a user is added (removed) from a group...

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What is the best approach to do that?? noting that i'm using it for licensing purpose and i want to know when the number of users in a specific group passed the maximum of the allowed number.

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This is an unexpected pain to implement: Refer to this previous Stack Overflow question or Aaron Han's answer in this social thread.