Sharing is enabled on all students on my roster but they do not have the option on their end. Please end.
Have them refresh their browser windows if you enabled the sharing in class while they were accessing their projects.
Some lessons hide the share buttons so that students cannot share until they are ready, especially in the beginning of the unit. To double check that students are allowed to share, they go to their projects page and create a new Web Lab project. If they are able to share that, then sharing is enabled.