controller

  def edit
    @publication = Publication.find(params[:id])
    @recipients = @publication.users
    @users = User.first
  end

view

<%= f.label :recipients %>
<select id='recipients_id' multiple='multiple' name='recipients[id][]'>
  <% User.all.each do |u| %>
    <option value='<%= u.id %>'
      <%= @recipients.include?(u) ? "selected='selected'" : '' %> ><%= u.name %></option>
  <% end %>
</select>

<%= collection_select(:users, :id, User.all, :id, :name, options ={}, {multiple: true}) %>

<%= select(:auser, 'userrec', User.all.map {|u| [u.name, u.id]}, {:selected => @recipients}, {multiple: true}) %>