<%= bootstrap_form_for(resource, as: resource_name, url: registration_path(resource_name)) do |f| %>
  <%= devise_error_messages! %>

  <div class="field">
    <%= f.email_field :email, autofocus: true %>
  </div>

  <div class="field">
    <% if @minimum_password_length %>
    <em>(<%= @minimum_password_length %> characters minimum)</em>
    <% end %><br />
    <%= f.password_field :password, autocomplete: "off" %>
  </div>

  <div class="field">
    <%= f.password_field :password_confirmation, autocomplete: "off" %>
  </div>

  <div class="actions">
    <%= f.submit "Join us" %>
  </div>
<% end %>

<%= bootstrap_form_for(resource, as: resource_name, url: registration_path(resource_name)) do |f| %>
  <%= devise_error_messages! %>

  <div class="field">
    <%= f.email_field :email, autofocus: true %>
  </div>

  <div class="field">
    <% if @minimum_password_length %>
    <em>(<%= @minimum_password_length %> characters minimum)</em>
    <% end %><br />
    <%= f.password_field :password, autocomplete: "off" %>
  </div>

  <div class="field">
    <%= f.password_field :password_confirmation, autocomplete: "off" %>
  </div>

  <div class="actions">
    <%= f.submit "Join us", :class => "btn custom-btn-1 btn-block btn-default" %>
  </div>
<% end %>

Modify 'Join us' button style
Modify 'Join us' button style
