Go to "Your Groups", and select "Add Students" from the top right hand corner.
Please note: you will only see the option to manually add students if your school or district has given you permission to "manually add students". If not, you will only be able to add existing students to a group.
Step 1: Select "Add Existing Students".
Step 2: Select the students you would like to add.
Step 3: Click "Add".