Following along in MyCase is recommended as you read the article.
Overview
Similar to email, MyCase lets you forward and share messages with other people. This is useful for when you would like to invite someone else into the conversation after the message was originally created.
Forward/Share a Message
Under the Communications tab, click Messages.
Select the message you would like to forward/share.
In the top right corner, click Invite Users.
Type the name of the person(s) you would like to share the message with, then click their name from the dropdown menu.
You can only share messages with firm users and clients who have MyCase access enabled.
Once you have added everyone that you'd like to share the message with, click Invite Users.
MyCase will then notify these people that a new message has been shared with them.
In the message thread itself, everyone will see that these people have been added to the conversation.