+add as contact
"I have, more the merrier!
If this is a genuine event then by posting about it openly on here is quite likely to prevent it taking place.
You are going to have either a Police presence or the boy racer crowd spoiling the fun for any doggers present.
If you wanted some more people to attend then give general details and have people pm you for more detailed information. You could look at the profiles of the people PMing you and you could decide if they seemed appropriate.