Enable or disable the permanent invitation link of their account from within your Admin dashboard:

Begin by selecting Permanent Invite from the 'Account Tools' dropdown on the left:

To Enable Permanent Link:

Click the "Enable Permanent Invite".

You can access the link from within the 'Account Tools' dropdown by clicking "Copy Link", or on your account 'Invitations' page within your 'Users' section.

From 'Users' Section:

Note: The permanent invitation link can only be used to add message senders, not admins or moderators. To make a user a moderator or an admin you can do so from within the 'Users' page. See: Managing User Permission Levels

To Disable Permanent Link:

Click "Delete" to turn off the permanent invitation link. 

Be advised that disabling your link cannot be undone. If you choose to re-enable your link, it will generate a brand new link. Any documentation where you might share the link with your senders will need to be updated.