I don't think they're doing that "invite people into the parade" anymore. Not since "Magical Moments" they haven't. I'm thinking the OP was wondering how guests can be Honorary Grand Marshals of parades.
From what I know, you've got to be in a family. All the HGM's I've seen have been addressed as "The <Blank> Family from <City, State>". That's #1. #2> Your family has to have children 12 and under. This is most likely Disney's way to promote how the perfect family is. Two parents, two children, all happy.
Having said this, it's safe to say that neither I nor my family will ever take part in a Disney parade.