As PP stated, all WDW buses can carry ecvs. They have room for 2 or 3 depending on the model of the bus.
Every bus stop has a painted rectangle on the ground marked with the HC symbol. This is where you park when waiting for the bus. It aligns with the back door of the bus, which is where the lift is located. The bus driver will open the back door and load you first, and secure it in place before opening the front door for the other guests. Your party can board with you. If your party is larger than 6, the remainder should board through the front with the rest of the guests. At your destination you will be off loaded last, after the other guests have departed.
HTH.