If you have already Created Groups and chose not to Auto Enroll users you will have to enroll them manually.
To enroll users manually, navigate to your Group Category, click the drop down next to the name and select Enroll Users.
To enroll a user in a group, select the check box in the Group Column next to the student name to enroll them in that group.
If you have many groups you can have it display one group at a time or you can use the search function to select specific students.
Click Save when done.