Create participation between a user and event model

Ok, I have 3 models, user event and participation, and the associations between these are:

class Event < ActiveRecord::Base

belongs_to :user
has_many :users
has_many :participations, through: :participations

...
end

and

class User < ActiveRecord::Base

has_many :events
has_many :participations
has_many :event_participations, through: :participations, :source => :event

...
end

and

class Participation < ActiveRecord::Base

belongs_to :user
belongs_to :event

...
end

and in my routes i have like( I'm aware that nested resources more than one level are not advised, but i don't know any other way around it):

  resources :user do
    resources :events do
      resources :participations
    end
  end

ok, until now on the event page, i can check if the current user is participating in that event or not, but i want to add a button to the event show page so if the user is not participating, it can do that easily, but i can't figure it out how to do that.
Thanks for your help.