Add the curly brackets for the hash. class is part of the last parameter. If you don't add the brackets, it goes as third
<%= f.select :manager_id, user.collect {|u| [u.username, u.id]}, {:prompt => "Select manager
username"}, {:class => "btn btn-default dropdown-toggle", :multiple => true} %>
I haven't tried the multiple option in select. But it should work according to this post