At the top of the group there should be an little + sign similar to the one in the Tennessee Educators group (image below) that says "+Join California Educators." Easy stuff.
Also I noticed you created TWO groups for your class - did you just mean to make one? I think I know what you were trying to do so I deleted the extra group and updated the one that has people in it. Let me know if that was cool
If you don't like the logo or need to rename the group you can! All you have to do is click on "edit group" - example our U Michigan group has a flag shot for the image.
You can also locate all your friends in the class and send them all an invite to join the group. You probably know this but I thought I'd put it out there!