To associate textbooks to your group, go to the "Contents" tab, in the group's page, and click on "Edit group".
You can also click on "Edit" on the top right corner of the group's page.
In the "Edit group" window, select in the "Contents" section the textbook or textbooks you want to work with in that group. If you have access to the full library, you can use the search bar and the "Sort by" option to locate them more easily.
Remember to click on "Save" at the bottom of the page to update the changes. Adding a textbook to the group doesn't automatically give the students access to its contents. Each one will have to activate the contents of the digital book from their own account in order to be able to access it.
When you return to the group's page, you will see the textbooks associated to it in the "Contents" tab.