It's sort of a "build it and they will come" kind of thing. Here on the forum we often just announce that so-and-so is going camping somewhere and invites others to join in. Not much planning other than to pick a place. Other rallies are more organized, with arrangements made with a campground for a group of sites, maybe a special group rate, and perhaps some activities, like a trip to a local attraction (museum, show, etc).
To invite people you don't ordinarily have have some contact, e.g. your neighborhood or city, with is an advertising problem. It's easier if there is an RV club or other way to reach prospective attendees. If you don't already have social contact with them, even indirectly thru a club, then you have to make the rally sound attractive enough to get their attention.