Free Accounts Can't Join Groups?/Offline?

I’m a teacher, and I specifically purchased a premium membership so that I could create a group for my class and have them all work on memorizing the same thing and create a leaderboard, etc. However, once I made the group and invited them to join, all of their accounts said they had to “go online” in order to be part of a group.

I also had this when I tried to create to go to “groups” on a free account, but figured that the free account was the issue, since everything else on my computer indicated I was online. This is when I checked on the Pricing, and saw that you can join up to 3 groups on a free account, so I decided to purchase a membership in order to create a group for my class.