1. Navigate to the Group option in the Community dropdown on the top menubar.

2. From the desired group block click Options --> Add Member

2. Select the member(s) (Use CTRL or SHIFT to choose multiple members) and then press the button pointing right.

3. Once the all desired members are moved from the "Available Members" to the "Chosen Members" section. Press the Save button. 

4. Once saved if all has gone well the group page should now show the added members