We were wondering if there was a way to provide a link in the email that will log the user into their profile.
Yes, that's possible using CB Auto Actions and its Login/Logout action. No, you should not do this.
Something secure that will expire after a single use (or something of that nature).
It will
never be secure. Again, you should not do this.
When going through CB auto actions, I see an option to log the user in automatically but that seems to not have a password.
The Login/Logout action can work with just username by leaving password blank, but again you should not do this.