class shGroupReference extends Model {

    protected $table      = 'shGroupReference';
    protected $primaryKey = 'groupID';
    public $timestamps    = false;
    protected $fillable   = ['referenceCode', 'referenceID', 'groupID'];

    public function shGroup() {
        return $this->belongsTo(\App\Models\shGroup::class, 'groupID', 'groupID');
    }
.....

class shGroup extends Model {

    protected $table      = 'shGroup';
    protected $primaryKey = 'groupID';
    public $timestamps    = false;
    protected $fillable   = ['groupID', 'orgID', 'name', 'categoryCode'];

    public function shGroupReference() {
        return $this->hasOne(\App\Models\shGroupReference::class, 'groupID', 'groupID');
    }
.....


$shGroupReference = App\Models\shGroupReference::create(['referenceCode' => 'caughtya', 'referenceID' => 1005]);
$shGroup          = App\Models\shGroup::create(['orgID' => 1, 'name' => 'blabla', 'categoryCode' => 'blublu']);
$shGroupReference->shGroup()->associate($shGroup)->save();

var_dump(shGroupReference->toArray());