The most likely cause is they haven't used the link (which in most cases serves as reference ID) to register their account. If they haven't received a personalized link in the email they received via your invite form, then there should be some other form of reference identification mentioned in that email. If not, then take it up with the admins of the game (as this most likely isn't a steam based issue, since the game itself is just hosted on steam but not produced nor maintaned by steam).

PS another possible reason is sharing the same public IP address (often the case in many ISP, especially localized to a building or few of them).