First, go to the group tab in essaypop, and click on the paper airplane icon of the class you'd like to invite.
This invite will appear. Simply copy and paste it to the messaging system of your choice.
Students go through the Google authentication process, just like you did, click the "student" button, then join your class by clicking the "join" tab. They will enter the join code into the blue "groups" box.
That's it! Your students are now enrolled in a designated class and they only have to do this once. Everything you assign will now show up automatically in their assignments page.