The players need to have used your invite link, and then joined the same world as the invite was sent from.
They also need to have not shared a connection with you at any point.
If the above has been adhered to, I suggest sending a support ticket in-game. You can do this by right-clicking anywhere on your game screen, then clicking 'Contact Support' From there you can fill in the details and file your ticket. Please make sure to give the names of your friends and as much detail as possible.