If you've take some time to look at the group, you've realized that you can read the messages people post without being a member.
But, you'll also notice there are some things reserved for Members Only. One of those things is access to the Files.
When you join the group, you'll magically gain access to those Files, and some other things as well.
And once you're in, you'll find folders and folders willed with lessons, activities, templates and more that others have created and uploaded to share with anyone in the group.
If you've been on the fence about whether to join or not, I highly recommend you do, both to access such a knowledgeable, supportive online community and the materials they're willing to share. And maybe you've even got something you're willing to share with the group in return!