## view [html_rails]
<% @events.each do |event| %>
  <%= event.name %>
  <%= button_to "Sign up!" event_event_signup_path(@event) %>
<% end %>

## routes.rb
map.resources :events do |event|
  event.resources :event_signups
end

## controller
class EventSignups < ApplicationController
  def create
    @event = Event.find(params[:event_id])
    @event_signup = @event.event_signups.new
    @event_signup.paparazzi = current_paparazzi # or current_user ?

    if @event_signup.save
      # ...
    else
      # ...
    end
  end
end