To add contacts to an existing group go to the "Contacts" module. From here select the contacts you wish to add to the existing group.
Once the contacts are selected click "Save for Later" and then choose "Add to Existing Group".
PRO TIP: If you don't want to scroll and find all the contacts at once simply search them by name, select them then click 'Save for Later' and add them to a your "Save for Later" list. Then once all your contacts are selected you can add them to an existing group all at once