File menu reference
File|Share
The Share command sets the other users who are also allowed to open the current markbook. Only the owner of the markbook can use this command. Select the name of a user in the Users list, and click on the Share button to move the name to the Share with list. Repeat this for all users who you want to share the markbook with. To remove a user from the Share with list, click on the name and then the Remove button.
Click on the Save Changes button when the list is complete.
Refer to the Working online topic in the Introduction section for more information about sharing markbooks.