diff --git a/app/assets/javascripts/discourse/controllers/invite.js.es6 b/app/assets/javascripts/discourse/controllers/invite.js.es6
index 7c7c7c17754..0dcf67c6957 100644
--- a/app/assets/javascripts/discourse/controllers/invite.js.es6
+++ b/app/assets/javascripts/discourse/controllers/invite.js.es6
@@ -43,15 +43,19 @@ export default ObjectController.extend(ModalFunctionality, {
 
   // Show Groups? (add invited user to private group)
   showGroups: function() {
-    return this.get('isAdmin') && (Discourse.Utilities.emailValid(this.get('emailOrUsername')) || this.get('isPrivateTopic') || !this.get('invitingToTopic'));
+    return this.get('isAdmin') && (Discourse.Utilities.emailValid(this.get('emailOrUsername')) || this.get('isPrivateTopic') || !this.get('invitingToTopic')) && !Discourse.SiteSettings.enable_sso;
   }.property('isAdmin', 'emailOrUsername', 'isPrivateTopic', 'invitingToTopic'),
 
   // Instructional text for the modal.
   inviteInstructions: function() {
-    if (this.get('isMessage')) {
+    if (Discourse.SiteSettings.enable_sso) {
+      // inviting existing user when SSO enabled
+      return I18n.t('topic.invite_reply.sso_enabled');
+    } else if (this.get('isMessage')) {
+      // inviting to a message
       return I18n.t('topic.invite_private.email_or_username');
     } else if (this.get('invitingToTopic')) {
-      // display instructions based on provided entity
+      // when inviting to topic, display instructions based on provided entity
       if (this.blank('emailOrUsername')) {
         return I18n.t('topic.invite_reply.to_topic_blank');
       } else if (Discourse.Utilities.emailValid(this.get('emailOrUsername'))) {
@@ -60,6 +64,7 @@ export default ObjectController.extend(ModalFunctionality, {
         return I18n.t('topic.invite_reply.to_topic_username');
       }
     } else {
+      // inviting to forum
       return I18n.t('topic.invite_reply.to_forum');
     }
   }.property('isMessage', 'invitingToTopic', 'emailOrUsername'),
@@ -76,15 +81,25 @@ export default ObjectController.extend(ModalFunctionality, {
   },
 
   successMessage: function() {
-    return this.get('isMessage') ?
-            I18n.t('topic.invite_private.success') :
-            I18n.t('topic.invite_reply.success', { emailOrUsername: this.get('emailOrUsername') });
+    if (this.get('isMessage')) {
+      return I18n.t('topic.invite_private.success');
+    } else if ( Discourse.Utilities.emailValid(this.get('emailOrUsername')) ) {
+      return I18n.t('topic.invite_reply.success_email', { emailOrUsername: this.get('emailOrUsername') });
+    } else {
+      return I18n.t('topic.invite_reply.success_username');
+    }
   }.property('isMessage', 'emailOrUsername'),
 
   errorMessage: function() {
     return this.get('isMessage') ? I18n.t('topic.invite_private.error') : I18n.t('topic.invite_reply.error');
   }.property('isMessage'),
 
+  placeholderKey: function() {
+    return Discourse.SiteSettings.enable_sso ?
+            'topic.invite_reply.username_placeholder' :
+            'topic.invite_private.email_or_username_placeholder';
+  }.property(),
+
   // Reset the modal to allow a new user to be invited.
   reset() {
     this.setProperties({
diff --git a/app/assets/javascripts/discourse/templates/modal/invite.hbs b/app/assets/javascripts/discourse/templates/modal/invite.hbs
index 16c80290d34..4ff86a04b3a 100644
--- a/app/assets/javascripts/discourse/templates/modal/invite.hbs
+++ b/app/assets/javascripts/discourse/templates/modal/invite.hbs
@@ -10,7 +10,7 @@
   {{else}}
     <label>{{inviteInstructions}}</label>
     {{#if allowExistingMembers}}
-      {{user-selector single="true" allowAny=true usernames=emailOrUsername includeGroups="true" placeholderKey="topic.invite_private.email_or_username_placeholder"}}
+      {{user-selector single="true" allowAny=true usernames=emailOrUsername includeGroups="true" placeholderKey=placeholderKey}}
     {{else}}
       {{text-field value=emailOrUsername placeholderKey="topic.invite_reply.email_placeholder"}}
     {{/if}}
diff --git a/app/models/invite.rb b/app/models/invite.rb
index 6b84554d2b5..51af2516650 100644
--- a/app/models/invite.rb
+++ b/app/models/invite.rb
@@ -66,7 +66,7 @@ class Invite < ActiveRecord::Base
     if topic.private_message?
       topic.grant_permission_to_user(user.email)
     elsif topic.category && topic.category.groups.any?
-      if Guardian.new(invited_by).can_invite_to?(topic)
+      if Guardian.new(invited_by).can_invite_to?(topic) && !SiteSetting.enable_sso
         (topic.category.groups - user.groups).each { |group| group.add(user) }
       end
     end
diff --git a/app/models/topic.rb b/app/models/topic.rb
index d48bf6ccc53..e2149238866 100644
--- a/app/models/topic.rb
+++ b/app/models/topic.rb
@@ -551,12 +551,12 @@ class Topic < ActiveRecord::Base
       end
     end
 
-    if username_or_email =~ /^.+@.+$/
+    if username_or_email =~ /^.+@.+$/ && !SiteSetting.enable_sso
       # NOTE callers expect an invite object if an invite was sent via email
       invite_by_email(invited_by, username_or_email, group_ids)
     else
       # invite existing member to a topic
-      user = User.find_by_username_or_email(username_or_email)
+      user = User.find_by_username(username_or_email)
       if user && topic_allowed_users.create!(user_id: user.id)
 
         # Notify the user they've been invited
diff --git a/config/locales/client.en.yml b/config/locales/client.en.yml
index 66f6b6429df..35e95293635 100644
--- a/config/locales/client.en.yml
+++ b/config/locales/client.en.yml
@@ -1073,15 +1073,18 @@ en:
 
       invite_reply:
         title: 'Invite'
+        username_placeholder: "username"
         action: 'Send Invite'
         help: 'invite others to this topic via email or notifications'
         to_forum: "We'll send a brief email allowing your friend to immediately join by clicking a link, no login required."
+        sso_enabled: "Enter the username of the person you'd like to invite to this topic."
         to_topic_blank: "Enter the username or email address of the person you'd like to invite to this topic."
         to_topic_email: "You've entered an email address. We'll email an invitation that allows your friend to immediately reply to this topic."
         to_topic_username: "You've entered a username. We'll send a notification to that user with a link inviting them to this topic."
 
         email_placeholder: 'name@example.com'
-        success: "We mailed out an invitation to <b>{{emailOrUsername}}</b>. We'll notify you when the invitation is redeemed. Check the invitations tab on your user page to keep track of your invites."
+        success_email: "We mailed out an invitation to <b>{{emailOrUsername}}</b>. We'll notify you when the invitation is redeemed. Check the invitations tab on your user page to keep track of your invites."
+        success_username: "We've invited that user to participate in this topic."
         error: "Sorry, we couldn't invite that person. Perhaps they have already been invited? (Invites are rate limited)"
 
       login_reply: 'Log In to Reply'
diff --git a/lib/guardian.rb b/lib/guardian.rb
index d3e2911ee1a..1db182aa8ed 100644
--- a/lib/guardian.rb
+++ b/lib/guardian.rb
@@ -197,12 +197,6 @@ class Guardian
     is_me?(user)
   end
 
-  def invitations_allowed?
-    !SiteSetting.enable_sso &&
-      SiteSetting.enable_local_logins &&
-      (!SiteSetting.must_approve_users? || is_staff?)
-  end
-
   def can_invite_to_forum?(groups=nil)
     authenticated? &&
     !SiteSetting.enable_sso &&
@@ -216,7 +210,7 @@ class Guardian
 
   def can_invite_to?(object, group_ids=nil)
     return false if ! authenticated?
-    return false if ! invitations_allowed?
+    return false unless ( SiteSetting.enable_local_logins && (!SiteSetting.must_approve_users? || is_staff?) )
     return true if is_admin?
     return false if ! can_see?(object)