Even if they share an account... normally once you log in, you stay logged in. You can be logged in on 100 computers with the same IP if you wanted. So really would only be a problem if they all decided to log out and then log in fresh at the same time (within an hour), *and* a bunch of them used the wrong password. The login attempt counter only counts if they use the wrong username/password.
They would all have to be logging in fresh (entering login/password) within a 30 minute window, rather than staying logged in. They would ALSO need to use the wrong login/password more than once. I'd say the chances of that happening multiple times in the short 30 minute window is pretty rare. And even *then*, the worst case scenario is they have to click the captcha button once (if you are using the "No CAPTCHA reCAPTCHA" option they don't even need to type anything to solve it).
Truthfully, I think you might be worrying about something that will never happen to a legit user... like I said, they would have to be getting their password wrong multiple times while logging in fresh within a short window of time to even be presented with the captcha option.
