Watch How to Search the Directory and Add Contacts to a Personal Contact Group in Google Contacts:
To add a contact to a contact group from the Directory:
- Select the Directory label.
- Search the Directory for the contact.
- Select the contact in the Contacts list.
- Click the Groups button.
- Select the group you'd like to add the contact to, or select Create new to create a new group.
Reference: Google Groups Help - Editing Contacts