Add site setting tos_accept_required. If enabled, users must check a box saying that they've read and accept the terms of service.

app/assets/javascripts/discourse/controllers/create_account_controller.js

@@ -17,6 +17,7 @@ Discourse.CreateAccountController = Discourse.Controller.extend(Discourse.ModalF
   rejectedEmails: Em.A([]),
   rejectedPasswords: Em.A([]),
   prefilledUsername: null,
+  tosAccepted: false,
 
   resetForm: function() {
     this.setProperties({
@@ -36,12 +37,13 @@ Discourse.CreateAccountController = Discourse.Controller.extend(Discourse.ModalF
 
   submitDisabled: function() {
     if (this.get('formSubmitted')) return true;
+    if (this.get('tosAcceptRequired') && !this.get('tosAccepted')) return true;
     if (this.get('nameValidation.failed')) return true;
     if (this.get('emailValidation.failed')) return true;
     if (this.get('usernameValidation.failed')) return true;
     if (this.get('passwordValidation.failed')) return true;
     return false;
-  }.property('nameValidation.failed', 'emailValidation.failed', 'usernameValidation.failed', 'passwordValidation.failed', 'formSubmitted'),
+  }.property('nameValidation.failed', 'emailValidation.failed', 'usernameValidation.failed', 'passwordValidation.failed', 'formSubmitted', 'tosAccepted'),
 
   passwordRequired: function() {
     return (this.blank('authOptions.auth_provider') || this.blank('authOptions.email_valid') || !this.get('authOptions.email_valid'));
@@ -322,6 +324,10 @@ Discourse.CreateAccountController = Discourse.Controller.extend(Discourse.ModalF
     });
   },
 
+  tosAcceptRequired: function() {
+    return Discourse.SiteSettings.tos_accept_required;
+  }.property(),
+
   actions: {
     createAccount: function() {
       var self = this;

app/assets/javascripts/discourse/templates/modal/create_account.js.handlebars

@@ -67,6 +67,12 @@
   </div>
 
   <div class="modal-footer">
+    {{#if tosAcceptRequired}}
+      <div class="tos-agree">
+        {{input type="checkbox" checked=tosAccepted}}
+        {{customHTML "tos_signup_form_message"}}
+      </div>
+    {{/if}}
     <button class='btn btn-large btn-primary' {{bind-attr disabled="submitDisabled"}} {{action createAccount}}>{{i18n create_account.title}}</button>
     {{#if formSubmitted}}
       &nbsp; <i class='fa fa-spinner fa-spin'></i>

app/assets/stylesheets/desktop/login.scss

@@ -32,6 +32,9 @@
 }
 
 .create-account {
+  form {
+    margin-bottom: 0;
+  }
   table {
     width: 100%;
   }
@@ -56,4 +59,8 @@
   .fa-spinner {
     font-size: 18px;
   }
+
+  .tos-agree {
+    margin-bottom: 12px;
+  }
 }