Joining a group is easy with a group code. On your groups page, click "Join a Group":
Enter the group code (the person who created the group will need to give you this code), then click "Join Group":
And that's it! You've joined the group: you'll be taken to the group page, where you can see your conversation with the group creator and begin adding files or feedback.
If you want to invite students to a group, have a look at this guide.