EMH is for all guests staying in a Disney hotel or those with Dream annual passes. Also another off site hotel but cant remember which.
Most days there will be either 2 extra hours before park opens or after closes.
When EMH is morning you can enter the park 2 hrs before official opening. Just before the bag search area there will be cast members checking hotel id and they only allow people through who are eligible. You just need to show id then can proceed through bag check and through main gates. I assume those who are eligible by having dream passes will show those instead.
There are then certain rides open for you -usually seems to be several in Fantasy land and a few in Discovery land.
About half hr before park officially opens they may let other guests in but they will only be allowed as far as end of main street then will have to wait for official rope drop. If you are eligible for EMH you can go from Fantasyland to D land and vv even though others are waiting for rope drop.
One tip is to do EMH then about 5 mins before park opens wait at rope drop in Fantasy land near Peterpan ride and be amongst first through to get on POC
In the evening you need to collect a wristband from near Main street Station from about 5.00 onwards. This allows you onto certain rides after the park officially closes. Again there tend to be rides open in Fantasyland and D land and a few extras such as POC, BTM.
Other guests are not made to leave and can still be around in the park in shops etc but are not allowed onto rides. The CMs check wristbands at the entrance to ride.
BTW- Only one member of party needs to go to collect wristbands but needs to show hotel id and park passes