Poppinga could get drafted, but probably not until 5th-7th round. His problem is that he isn't quite big enough for ILB nor fast enough for OLB. He is like Jensen, who was a great linebacker, but not the right size. Jensen is sticking on practice squads so there is a shot for Poppinga (not to mention he has a brother in the NFL so that will get him some looks.)
Invites are based on who scouts/combine organizers consider top-flight draft prospects. You can still get drafted (Pro Day in March to show off) but its much harder.