From the Admin Menu (icons on the left side of your Dashboard), follow this path:
Admin Menu > Users&Groups > Groups > Search for your group > Group Properties > Group Members > Add Users > Search Member > Highlight desired Search result > Submit
From the Admin Menu (icons on the left side of your Dashboard), follow this path:
Admin Menu > Users&Groups > Groups > Search for your group > Group Properties > Group Members > Add Users > Search Member > Highlight desired Search result > Submit