Great question. I do not know the answer with 100% accuracy. I wondered about this as well. I think on the WDW site, from my limited research on the subject, the best info I got was, "offsite guests may stay inside the park, but they are not permitted to experience attractions" but that was for evening EMH. What I experienced for AM EMH, at both HS and EP however, was this:
It appeared as though offsite guests were not allowed into the park. I didn't stick around and make SURE, but at both parks, I saw one person trying to get the green light at the tapstyle, but being denied. After a few tries, the cm then politely asked if they were staying at a Disney property, and started into an explanation of the EMH. Again, I didn't stick around and watch/listen to make 100% sure, so take this with a grain of salt. Not sure if it's explained fully on the Dis site.