You have 2 options to do so:
1. You can directly contact the group admin and ask them to add you to the group. The admin is usually the teacher of the course group, the class teacher, or the creator of the group.
2. The group admin gives you an "access code", which you can enter under Groups. To do so click on "My groups" in the drop down menu at the top right in the header. Then select "join group with code".
Enter the code into the grey field and then click on "Join".