A ECV can be brought into most lines thru the same line as other guests. You may sometimes be directed to a different boarding area, but will usually be waiting in line with the other guests until almost at boarding.
If you are directed to go somewhere else, the rat of your party can go with you, up to a total of 6 (5 plus the person with a disability).
Some lines are not ECV accessible, but all are wheelchair accessible (except for Tomorrowland Transit Authority, Tom Sawyer's Island and the Swiss Family Robinson Treehouse, where guests must be ambulatory). Lines that are not ECV accessible will have a wheelchair you can transfer to for use in the line.
You will find a LOT more information in the disABILITIES FAQs thread, located near the top of this board or follow the link in my signature.