Well...as a former CM and recent visitor to WDW, the process has now changed. It used to be that any company ID would get you into the parks without using a Sign-in on your main gate. However, you are no longer able to do that. The WDW ID should get you parking at Mickey and Friends Parking Structure ONLY. The other lots will not get complimentary parking.
Also, for entrance to the parks, they will have to use their Main Gate. However, it will not be stamped (as is the same in WDW for DL CM's). If they start to stamp it, be sure to have them stop and let them know it's a WDW Main Gate. They are not allowed to come in on just their ID.
Hope it helps....