Students need to contact each other. At your next virtual class, read off a list of your students before Covid-19. Ask students that are attending your virtual class to call their friends. Tell them there is an award for all of them and a special award for the one that gets their whole list to attend. I would not ask them to contact more than five students. Awards can be in the way of anything you can think of. Allow your virtual students to pick the students they want to call by reading from the list and allowing your virtual students to interact with each other. Give the phone numbers to your students. There needs to be a special reason everyone wants to come together so much that they will make sure Mom gets them on zoom. A raffle, a tournament a cause with a great prize. The object is to teach then that they are really missing their friends and they want to be together in class.