I get this request from my team mates fairly often, so I figured I should pass it on. Everyone has weird ideas on how to implement it, and I don't know what would work, but the question usually goes something like this:
'Can you put a button in the email that lets us RSVP for events without logging in?'
I don't consider typing a username and password much of a time commitment, but judging by the large quantity of regular "No Response" team members we have, something different in the email might be warranted. I don't know if a fancy mailto link could work, or maybe something cool I don't know about in an HTML email.