To join an existing group, you have two options:

Group Admin
You can contact the group admin directly and ask them to add you to the group. This is usually the teacher of the respective course group, the class teacher, or the creator of the group.

Group join code
You can get the “Group Join Code” from the group admin. You use this as follows:

Go to “My Groups” using the drop-down menu at the top right of the header. Then select the item “Join group with code”.

In the window that opens, enter the code in the gray “Insert group code” field and then click “Join”.