We've done this twice -- had 8 am ADRs at CP on days when MK opened at 9. They'll have you line up at one of the turnstyles (both times, it has been to the far left as you face the park, but that could certainly change) and at juuuuust about 8 (maybe a few minutes before, but not much) they'll start letting you in. It has certainly never been more than a couple of minutes before 8, and then CP does a great job of getting you in and seated.
The good CMs at your resort will tell you exactly when the monorail starts -- I *think* it's generally 7:30 a.m., but in any case it is certainly early enough to get monorail resort guests out to their 8 a.m. ADRs on time.
Enjoy!