You can only add team members to a folder if it is visible to the Team. Private folders are visible to you only. If your folder is private, you can change the visibility by following this article.
You might be wondering, why do I need to add members to clients?
Only account members will receive specific recommendations and notifications related to this account - such as weekly update emails - and will be able to view these accounts in the "My Clients" shortcut tab of the Dashboard.
To add members, hit the light blue "Edit" button in the top right corner of the page.
In the dialog box that appears next to "Members", amend the account members by adding/removing the relevant people and hitting the light blue "Submit" button.