To listen to membership changes, you need to extend org.exoplatform.services.organization.MembershipEventListener :

public class MyMembershipListener extends MembershipEventListener {


  public void preSave(Membership membership, boolean isNew) throws Exception {

    System.out.println("Before " + (isNew?"creating":"updating") + " membership.");

  }


  public void postSave(Membership membership, boolean isNew) throws Exception { 

    System.out.println("After membership " + (isNew?" created":" updated"));

  }


  public void preDelete(Membership membership) throws Exception {

    System.out.println("Before deleting membership");

  }


  public void preDelete(Membership membership) throws Exception {

    System.out.println("After deleting membership");

  }

}