Signing up as crew allows them to work on the car. If they help fuel, yes, they need full suit and helmet unless you plan on only fueling at the pumps (assuming your track has them), where gear isn't required. They can borrow a team mates for fueling if everyone involved is cool with the gross sweaty factor, but I like not sharing gear personally.
If they don't even want to work on the car, just be there and watch, they can just show up and pay the spectator $30 for the weekend. But again, can't wrench on the car as a spectator. Can't touch a fuel jug, can't touch the car, can't move wheels around, etc. But spectators can hang out in the pits all weekend. If you think they want to help with the car, sign them up as crew.
You can put almost anything on the outside of the car as long as it meets two requirements
1. It doesn't completely block some part of your vision
2. it's attached well enough that 100mph wind isn't going to tear it off.
20+ Time Loser
FutilityMotorsport Abandoned E36 Build2008 Saab 9-5Aero WagonRetired -
1989 Dodge Daytona Shelby 2011-2015 "Lifetime Award for Lack of Achievement" IOE, 3X I got screwed, Organizer's Choice