Invitations can only be sent and resent through the Administrator tool using the following steps:

1. Log into the Administrator tool
2. In the left hand TreeView pane Expand the "User Rights Assignment" node 
3. Left Click on the "Administrators" group. 
4. Locate the user you want to send an invite to in the right hand content pane 
5. Right Click on the user's name (or hold down the ctrl/shift keys to select multiple users at the same time). 
6. Choose “Invite Administrator…” in the context menu to send out new invitations.