Guests can use the "forward" link on their invitation to forward to others. A guest who received the invitation from a forward (the "forwardee") will show up on your guest list with "FWD" in the Sent column.
If you want to turn off the forward link on your invitation, go to your Invitation Options page and choose "OFF" for "Allow guests to forward the invitation".
Note: If a guest forwards the actual email that contains the invitation (instead of using the Forward Invitation link) they are essentially giving someone else their invite. Anyone they forward to will be RSVPing under the forwarder's name instead of their own name.