As an 18 year old, you are able to check in to a WDW resort. You do not need anyone that is 21+.
Please note, that you will be responsible for the responsibility of the minors. This includes paying for any damages, underage drinking, shoplifting, or anything else including medical decisions.
Do you feel your friends are responsible enough that without parental supervision, they won't go crazy?
Before your trip, you might want to have all 6 of you and all parents sign something stating any extra charges for damages will be equally shared amongst you.
Also it would be a good idea for the minors to have letters saying that you are acting as their chaperon and legal guardian while on the trip.