It happens often that a tester can miss a test inviation and when the tester sees the notification there are no more slots in this test and it dissapears from the dashboard.Anyways after a while a slot can be avaliable and the test inviation appears in dashboard.

I suggest that a notification will be sent (by email and Testnow app) to the testers who missed the invitation when a slot becomes avaliable and the invitation is shown in dashboard, in order to accept the test invitation immediately.