diff --git a/app/assets/javascripts/discourse/components/composer-editor.js.es6 b/app/assets/javascripts/discourse/components/composer-editor.js.es6
index 9cc3120bb16..51a7a8fe143 100644
--- a/app/assets/javascripts/discourse/components/composer-editor.js.es6
+++ b/app/assets/javascripts/discourse/components/composer-editor.js.es6
@@ -114,6 +114,25 @@ export default Ember.Component.extend({
   _renderUnseen: function($preview, unseen) {
     fetchUnseenMentions($preview, unseen, this.siteSettings).then(() => {
       linkSeenMentions($preview, this.siteSettings);
+      this._warnMentionedGroups($preview);
+    });
+  },
+
+  _warnMentionedGroups($preview) {
+    Ember.run.scheduleOnce('afterRender', () => {
+      this._warnedMentions = this._warnedMentions || [];
+      var found = [];
+      $preview.find('.mention-group.notify').each((idx,e) => {
+        const $e = $(e);
+        var name = $e.data('name');
+        found.push(name);
+        if (this._warnedMentions.indexOf(name) === -1){
+          this._warnedMentions.push(name);
+          this.sendAction('groupsMentioned', [{name: name, user_count: $e.data('mentionable-user-count')}]);
+        }
+      });
+
+      this._warnedMentions = found;
     });
   },
 
@@ -370,6 +389,8 @@ export default Ember.Component.extend({
         Ember.run.debounce(this, this._renderUnseen, $preview, unseen, 500);
       }
 
+      this._warnMentionedGroups($preview);
+
       const post = this.get('composer.post');
       let refresh = false;
 
diff --git a/app/assets/javascripts/discourse/controllers/composer-messages.js.es6 b/app/assets/javascripts/discourse/controllers/composer-messages.js.es6
index 345837474f2..7a712ba7487 100644
--- a/app/assets/javascripts/discourse/controllers/composer-messages.js.es6
+++ b/app/assets/javascripts/discourse/controllers/composer-messages.js.es6
@@ -48,6 +48,23 @@ export default Ember.ArrayController.extend({
     this.get('queuedForTyping').forEach(msg => this.send("popup", msg));
   },
 
+  groupsMentioned(groups) {
+    // reset existing messages, this should always win it is critical
+    this.reset();
+    groups.forEach(group => {
+      const msg = I18n.t('composer.group_mentioned', {
+        group: "@" + group.name,
+        count: group.user_count,
+        group_link: Discourse.getURL(`/group/${group.name}/members`)
+      });
+      this.send("popup",
+        Em.Object.create({
+          templateName: 'composer/group-mentioned',
+          body: msg})
+        );
+    });
+  },
+
   // Figure out if there are any messages that should be displayed above the composer.
   queryFor(composer) {
     if (this.get('checkedMessages')) { return; }
diff --git a/app/assets/javascripts/discourse/controllers/composer.js.es6 b/app/assets/javascripts/discourse/controllers/composer.js.es6
index d4b15a0af70..822c833010c 100644
--- a/app/assets/javascripts/discourse/controllers/composer.js.es6
+++ b/app/assets/javascripts/discourse/controllers/composer.js.es6
@@ -171,6 +171,12 @@ export default Ember.Controller.extend({
       }
     },
 
+    groupsMentioned(groups) {
+      if (!this.get('model.creatingPrivateMessage') && !this.get('model.topic.isPrivateMessage')) {
+        this.get('controllers.composer-messages').groupsMentioned(groups);
+      }
+    }
+
   },
 
   categories: function() {
diff --git a/app/assets/javascripts/discourse/lib/link-mentions.js.es6 b/app/assets/javascripts/discourse/lib/link-mentions.js.es6
index 0dfa36adba2..476ae64fc45 100644
--- a/app/assets/javascripts/discourse/lib/link-mentions.js.es6
+++ b/app/assets/javascripts/discourse/lib/link-mentions.js.es6
@@ -1,8 +1,13 @@
 function replaceSpan($e, username, opts) {
   if (opts && opts.group) {
+    var extra = "", extraClass = "";
+    if (opts.mentionable) {
+      extra = " data-name='" + username + "' data-mentionable-user-count='" + opts.mentionable.user_count + "' ";
+      extraClass = " notify";
+    }
     $e.replaceWith("<a href='" +
                   Discourse.getURL("/groups/") + username +
-                  "' class='mention-group'>@" + username + "</a>");
+                  "' class='mention-group" + extraClass + "'" + extra + ">@" + username + "</a>");
   } else {
     $e.replaceWith("<a href='" +
                   Discourse.getURL("/users/") + username.toLowerCase() +
@@ -12,6 +17,7 @@ function replaceSpan($e, username, opts) {
 
 const found = [];
 const foundGroups = [];
+const mentionableGroups = [];
 const checked = [];
 
 function updateFound($mentions, usernames) {
@@ -22,7 +28,8 @@ function updateFound($mentions, usernames) {
       if (found.indexOf(username.toLowerCase()) !== -1) {
         replaceSpan($e, username);
       } else if (foundGroups.indexOf(username) !== -1) {
-        replaceSpan($e, username, {group: true});
+        const mentionable = _(mentionableGroups).where({name: username}).first();
+        replaceSpan($e, username, {group: true, mentionable: mentionable});
       } else if (checked.indexOf(username) !== -1) {
         $e.addClass('mention-tested');
       }
@@ -48,6 +55,8 @@ export function fetchUnseenMentions($elem, usernames) {
   return Discourse.ajax("/users/is_local_username", { data: { usernames } }).then(function(r) {
     found.push.apply(found, r.valid);
     foundGroups.push.apply(foundGroups, r.valid_groups);
+    mentionableGroups.push.apply(mentionableGroups, r.mentionable_groups);
     checked.push.apply(checked, usernames);
+    return r;
   });
 }
diff --git a/app/assets/javascripts/discourse/templates/composer.hbs b/app/assets/javascripts/discourse/templates/composer.hbs
index 0777937800f..bc3fa3270f5 100644
--- a/app/assets/javascripts/discourse/templates/composer.hbs
+++ b/app/assets/javascripts/discourse/templates/composer.hbs
@@ -82,6 +82,7 @@
                           canWhisper=canWhisper
                           draftStatus=model.draftStatus
                           isUploading=isUploading
+                          groupsMentioned="groupsMentioned"
                           importQuote="importQuote"
                           showOptions="showOptions"
                           showUploadSelector="showUploadSelector"}}
diff --git a/app/assets/javascripts/discourse/templates/composer/group-mentioned.hbs b/app/assets/javascripts/discourse/templates/composer/group-mentioned.hbs
new file mode 100644
index 00000000000..b59da973c3e
--- /dev/null
+++ b/app/assets/javascripts/discourse/templates/composer/group-mentioned.hbs
@@ -0,0 +1,2 @@
+<a href {{action "closeMessage" this}} class='close'><i class='fa fa-times-circle'></i></a>
+{{{body}}}
diff --git a/app/controllers/users_controller.rb b/app/controllers/users_controller.rb
index 8c28d3f1883..213442d11d2 100644
--- a/app/controllers/users_controller.rb
+++ b/app/controllers/users_controller.rb
@@ -206,13 +206,21 @@ class UsersController < ApplicationController
     usernames.each(&:downcase!)
 
     groups = Group.where(name: usernames).pluck(:name)
+    mentionable_groups =
+      if current_user
+        Group.mentionable(current_user)
+          .where(name: usernames)
+          .pluck(:name, :user_count)
+          .map{ |name,user_count| {name: name, user_count: user_count} }
+      end
+
     usernames -= groups
 
     result = User.where(staged: false)
                  .where(username_lower: usernames)
                  .pluck(:username_lower)
 
-    render json: {valid: result, valid_groups: groups}
+    render json: {valid: result, valid_groups: groups, mentionable_groups: mentionable_groups}
   end
 
   def render_available_true
diff --git a/config/locales/client.en.yml b/config/locales/client.en.yml
index e8b5454a956..e66c1f214e4 100644
--- a/config/locales/client.en.yml
+++ b/config/locales/client.en.yml
@@ -850,6 +850,8 @@ en:
       similar_topics: "Your topic is similar to..."
       drafts_offline: "drafts offline"
 
+      group_mentioned: "By using {{group}}, you are about to notify <a href='{{group_link}}'>{{count}} people</a>."
+
       error:
         title_missing: "Title is required"
         title_too_short: "Title must be at least {{min}} characters"