Additional plugins are membership and marketpress.
If you have a guest user (visitor)and they want to register for an event, the "I'm attending" passes to wp-login (ugly) wordpress login but not register bec. membership says to disable "anyone can register". Let the I'm attending call the nice pop-up membership applet that has both login and register on it.
Additionally, if the user is logged in, why have them "I'm attending", why can't they simply see an add to cart(marketpress integration) and it passes to the event and rsvp's.