CollaboratorsController.rb
============================
class CollaboratorsController < ApplicationController
   before_action :authenticate_user!

   def create
     wiki = Wiki.find(params[:wiki_id])
     collaborator = current_user.collaborators.build(wiki: wiki)

     if collaborator.save
       flash[:notice] = "Collaborator created"
     else
       flash[:alert] = "Collaborator creation failed."
     end
     redirect_to wikis_path
   end

   def destroy
     wiki = Wiki.find(params[:id])
     collaborator = current_user.collaborators.where(wiki: wiki).create

     if collaborator.destroy
       flash[:notice] = "Removed collaborator."
     else
       flash[:alert] = "Collaborator removal failed."
     end
       redirect_to wikis_path
   end
end

_collaborator.html.erb
==========================
<div class="form-group">
    <%# this should more properly use a variable set in your controller rather than User.all - for example you might want to limit the list of possible collaborators according to some condition %>
    <% User.all.each do |user| %>
      <% #if collaborator = user.collaborator_for(wiki) %>
      <% if wiki.private? && user != wiki.user && !wiki.collaborators.include?(user) %>
        <div class="user">
          <%= user.name %>
          <%= link_to  "make collaborator", collaborators_path(wiki_id: wiki.id), class: 'btn btn-primary', method: :post %>
        </div>
      <% end %>
      <% if wiki.private? && user != wiki.user && wiki.collaborators.include?(user) %>
        <%= user.name %>
        <%= link_to "Remove_collaborator", collaborator_path(wiki), class: 'btn btn-danger', method: :delete %>
        <%#= link_to "delete collaborator", [wiki, collaborator], method: :delete do %>
      <% end %>
    <% end %>
  </div>

  rails console output:
  =======================
  sharadalt:~/workspace (wiki_collaborators) $ rails c
  [Simple Form] Simple Form is not configured in the application and will use the default values. Use `rails generate simple_form:install` to generate the Simple Form configuration.
  Loading development environment (Rails 4.2.4)
  [1] pry(main)> wiki = Wiki.last
    Wiki Load (0.3ms)  SELECT  "wikis".* FROM "wikis"  ORDER BY "wikis"."id" DESC LIMIT 1
  => #<Wiki:0x00000005dbb5f0
   id: 11,
   title: "wiki user2",
   body: "bbbbbbbb",
   private: true,
   user_id: 2,
   created_at: Wed, 30 Mar 2016 06:16:22 UTC +00:00,
   updated_at: Wed, 30 Mar 2016 06:16:22 UTC +00:00>
  [2] pry(main)> wiki.user
    User Load (0.4ms)  SELECT  "users".* FROM "users" WHERE "users"."id" = ? LIMIT 1  [["id", 2]]
  => #<User:0x00000005d366c0
   id: 2,
   name: "user2",
   email: "user2@example.com",
   encrypted_password: "$2a$10$qREytQJfrjMp.O4dvuHaMuARBs4ZOdCyM/1yjxUonGneEz8NjobOW",
   reset_password_token: nil,
   reset_password_sent_at: nil,
   remember_created_at: nil,
   sign_in_count: 1,
   current_sign_in_at: Wed, 30 Mar 2016 06:15:05 UTC +00:00,
   last_sign_in_at: Wed, 30 Mar 2016 06:15:05 UTC +00:00,
   current_sign_in_ip: "69.181.65.0",
   last_sign_in_ip: "69.181.65.0",
   confirmation_token: "KogTgcBwNGGcDfhyDWuz",
   confirmed_at: Wed, 30 Mar 2016 05:54:19 UTC +00:00,
   confirmation_sent_at: Wed, 30 Mar 2016 05:54:18 UTC +00:00,
   unconfirmed_email: nil,
   failed_attempts: 0,
  [3] pry(main)>
  [4] pry(main)> wiki.collaborators
    Collaborator Load (0.3ms)  SELECT "collaborators".* FROM "collaborators" WHERE "collaborators"."wiki_id" = ?  [["wiki_id", 11]]
  => [#<Collaborator:0x00000005a842d8
    id: 1,
    user_id: 2,
    wiki_id: 11,
    created_at: Wed, 30 Mar 2016 06:16:31 UTC +00:00,
    updated_at: Wed, 30 Mar 2016 06:16:31 UTC +00:00>]
  [5] pry(main)> current_user
  NameError: undefined local variable or method `current_user' for main:Object
  from (pry):4:in `__pry__'
  [6] pry(main)> wiki.user
  => #<User:0x00000005d366c0
   id: 2,
   name: "user2",
   email: "user2@example.com",
   encrypted_password: "$2a$10$qREytQJfrjMp.O4dvuHaMuARBs4ZOdCyM/1yjxUonGneEz8NjobOW",
   reset_password_token: nil,
   reset_password_sent_at: nil,
   remember_created_at: nil,
   sign_in_count: 1,
   current_sign_in_at: Wed, 30 Mar 2016 06:15:05 UTC +00:00,
   last_sign_in_at: Wed, 30 Mar 2016 06:15:05 UTC +00:00,
   current_sign_in_ip: "69.181.65.0",
   last_sign_in_ip: "69.181.65.0",
  [7] pry(main)> wiki.user.collaborators
    Collaborator Load (0.3ms)  SELECT "collaborators".* FROM "collaborators" WHERE "collaborators"."user_id" = ?  [["user_id", 2]]
  => [#<Collaborator:0x00000005904fc0
    id: 1,
    user_id: 2,
    wiki_id: 11,
    created_at: Wed, 30 Mar 2016 06:16:31 UTC +00:00,
    updated_at: Wed, 30 Mar 2016 06:16:31 UTC +00:00>]
[8] pry(main)>