How to Host a Rally

to host a Rally;

You need a campground or park that you can host the approximate number of camper you expect.
Contact the management and check to see if they will hold some number of sites for your rally.
This will make sure no one that wants to attend is left out. Agree with the park manager on  a date when they can release the untaken sites. This way everyone is happy and part of the process.

Find one that has facilities that will attract the campers. Full hookups for electric, water and maybe sewage would be great. Many can deal with lack of sewage but the electric and water are a must.
WiFi is a great and desirable option, as is cable TV. Sure this is camping, but folks want their creature comforts.
If you decide to go without these options, your group may enjoy the serenity. Just sayin'...

I prefer parks with a pool or lake. Water is a draw and will make the event even more fun.