method to post a feed item to a group that mentions a
user.
ConnectApi.FeedItemInput feedItemInput = new ConnectApi.FeedItemInput();
ConnectApi.MentionSegmentInput mentionSegmentInput = new ConnectApi.MentionSegmentInput();
ConnectApi.MessageBodyInput messageBodyInput = new ConnectApi.MessageBodyInput();
ConnectApi.TextSegmentInput textSegmentInput = new ConnectApi.TextSegmentInput();
messageBodyInput.messageSegments = new List<ConnectApi.MessageSegmentInput>();
mentionSegmentInput.id = '005RR000000Dme9';
messageBodyInput.messageSegments.add(mentionSegmentInput);
textSegmentInput.text = 'Could you take a look?';
messageBodyInput.messageSegments.add(textSegmentInput);
feedItemInput.body = messageBodyInput;
feedItemInput.feedElementType = ConnectApi.FeedElementType.FeedItem;
feedItemInput.subjectId = '0F9RR0000004CPw';
ConnectApi.FeedElement feedElement = ConnectApi.ChatterFeeds.postFeedElement(Network.getNetworkId(), feedItemInput, null);