diff --git a/app/components/post_add_channel_member/post_add_channel_member.js b/app/components/post_add_channel_member/post_add_channel_member.js
index 1c4b89cbf..3a8dba028 100644
--- a/app/components/post_add_channel_member/post_add_channel_member.js
+++ b/app/components/post_add_channel_member/post_add_channel_member.js
@@ -30,6 +30,7 @@ export default class PostAddChannelMember extends React.PureComponent {
         postId: PropTypes.string.isRequired,
         userIds: PropTypes.array.isRequired,
         usernames: PropTypes.array.isRequired,
+        noGroupsUsernames: PropTypes.array,
         navigator: PropTypes.object.isRequired,
         onPostPress: PropTypes.func,
         textStyles: PropTypes.object,
@@ -78,7 +79,7 @@ export default class PostAddChannelMember extends React.PureComponent {
         }
     }
 
-    generateAtMentions(usernames = []) {
+    generateAtMentions(usernames = [], textStyles) {
         if (usernames.length === 1) {
             return (
                 <AtMention
@@ -95,6 +96,7 @@ export default class PostAddChannelMember extends React.PureComponent {
                         key={key}
                         id={'post_body.check_for_out_of_channel_mentions.link.and'}
                         defaultMessage={' and '}
+                        style={textStyles}
                     />
                 );
             }
@@ -134,7 +136,7 @@ export default class PostAddChannelMember extends React.PureComponent {
     }
 
     render() {
-        const {channelType, baseTextStyle, postId, usernames} = this.props;
+        const {channelType, baseTextStyle, postId, usernames, noGroupsUsernames} = this.props;
 
         if (!postId || !channelType) {
             return null;
@@ -150,43 +152,75 @@ export default class PostAddChannelMember extends React.PureComponent {
             linkText = 'add them to the channel';
         }
 
-        let messageId;
-        let messageText;
+        let outOfChannelMessageID;
+        let outOfChannelMessageText;
+        const outOfChannelAtMentions = this.generateAtMentions(usernames, baseTextStyle);
         if (usernames.length === 1) {
-            messageId = t('post_body.check_for_out_of_channel_mentions.message.one');
-            messageText = 'was mentioned but is not in the channel. Would you like to ';
+            outOfChannelMessageID = t('post_body.check_for_out_of_channel_mentions.message.one');
+            outOfChannelMessageText = 'was mentioned but is not in the channel. Would you like to ';
         } else if (usernames.length > 1) {
-            messageId = t('post_body.check_for_out_of_channel_mentions.message.multiple');
-            messageText = 'were mentioned but they are not in the channel. Would you like to ';
+            outOfChannelMessageID = t('post_body.check_for_out_of_channel_mentions.message.multiple');
+            outOfChannelMessageText = 'were mentioned but they are not in the channel. Would you like to ';
         }
 
-        const atMentions = this.generateAtMentions(usernames);
+        let outOfGroupsMessageID;
+        let outOfGroupsMessageText;
+        const outOfGroupsAtMentions = this.generateAtMentions(noGroupsUsernames, baseTextStyle);
+        if (noGroupsUsernames?.length) {
+            outOfGroupsMessageID = t('post_body.check_for_out_of_channel_groups_mentions.message');
+            outOfGroupsMessageText = 'did not get notified by this mention because they are not in the channel. They are also not a member of the groups linked to this channel.';
+        }
 
-        return (
-            <Text>
-                {atMentions}
-                {' '}
-                <FormattedText
-                    id={messageId}
-                    defaultMessage={messageText}
-                    style={baseTextStyle}
-                />
-                <Text
-                    style={this.props.textStyles.link}
-                    id='add_channel_member_link'
-                    onPress={this.handleAddChannelMember}
-                >
+        let outOfChannelMessage = null;
+        if (usernames.length) {
+            outOfChannelMessage = (
+                <Text>
+                    {outOfChannelAtMentions}
+                    {' '}
                     <FormattedText
-                        id={linkId}
-                        defaultMessage={linkText}
+                        id={outOfChannelMessageID}
+                        defaultMessage={outOfChannelMessageText}
+                        style={baseTextStyle}
+                    />
+                    <Text
+                        style={this.props.textStyles.link}
+                        id='add_channel_member_link'
+                        onPress={this.handleAddChannelMember}
+                    >
+                        <FormattedText
+                            id={linkId}
+                            defaultMessage={linkText}
+                        />
+                    </Text>
+                    <FormattedText
+                        id={'post_body.check_for_out_of_channel_mentions.message_last'}
+                        defaultMessage={'? They will have access to all message history.'}
+                        style={baseTextStyle}
                     />
                 </Text>
-                <FormattedText
-                    id={'post_body.check_for_out_of_channel_mentions.message_last'}
-                    defaultMessage={'? They will have access to all message history.'}
-                    style={baseTextStyle}
-                />
-            </Text>
+            );
+        }
+
+        let outOfGroupsMessage = null;
+        if (noGroupsUsernames?.length) {
+            outOfGroupsMessage = (
+                <Text>
+                    {outOfGroupsAtMentions}
+                    {' '}
+                    <FormattedText
+                        id={outOfGroupsMessageID}
+                        defaultMessage={outOfGroupsMessageText}
+                        style={baseTextStyle}
+                    />
+                </Text>
+            );
+        }
+
+        return (
+            <>
+                {outOfChannelMessage}
+                {outOfGroupsMessage}
+            </>
         );
     }
 }
diff --git a/app/components/post_body/post_body.js b/app/components/post_body/post_body.js
index 344fdf43e..62d22b763 100644
--- a/app/components/post_body/post_body.js
+++ b/app/components/post_body/post_body.js
@@ -194,6 +194,16 @@ export default class PostBody extends PureComponent {
             PostAddChannelMember = require('app/components/post_add_channel_member').default;
         }
 
+        let userIds = postProps.add_channel_member.not_in_channel_user_ids;
+        let usernames = postProps.add_channel_member.not_in_channel_usernames;
+
+        if (!userIds) {
+            userIds = postProps.add_channel_member.user_ids;
+        }
+        if (!usernames) {
+            usernames = postProps.add_channel_member.usernames;
+        }
+
         return (
             <PostAddChannelMember
                 baseTextStyle={messageStyle}
@@ -201,8 +211,9 @@ export default class PostBody extends PureComponent {
                 onPostPress={onPress}
                 textStyles={textStyles}
                 postId={postProps.add_channel_member.post_id}
-                userIds={postProps.add_channel_member.user_ids}
-                usernames={postProps.add_channel_member.usernames}
+                userIds={userIds}
+                usernames={usernames}
+                noGroupsUsernames={postProps.add_channel_member.not_in_groups_usernames}
             />
         );
     };
diff --git a/assets/base/i18n/en.json b/assets/base/i18n/en.json
index 40ea8508f..bd8dae740 100644
--- a/assets/base/i18n/en.json
+++ b/assets/base/i18n/en.json
@@ -462,6 +462,7 @@
   "password_send.link": "If the account exists, a password reset email will be sent to:",
   "password_send.reset": "Reset my password",
   "permalink.error.access": "Permalink belongs to a deleted message or to a channel to which you do not have access.",
+  "post_body.check_for_out_of_channel_groups_mentions.message": "did not get notified by this mention because they are not in the channel. They are also not a member of the groups linked to this channel.",
   "post_body.check_for_out_of_channel_mentions.link.and": " and ",
   "post_body.check_for_out_of_channel_mentions.link.private": "add them to this private channel",
   "post_body.check_for_out_of_channel_mentions.link.public": "add them to the channel",
@@ -552,4 +553,4 @@
   "user.settings.push_notification.offline": "Offline",
   "user.settings.push_notification.online": "Online, away or offline",
   "web.root.signup_info": "All team communication in one place, searchable and accessible anywhere"
-}
+}
\ No newline at end of file