This is overwritten if GroupsController::validateGroupEmail doesn't fail, so it seems unnecessary.

This overwrite errors if GroupsController::validateGroupEmail already failed. Either merge the errors or don't further validate after the first error.

I would probably just integrate GroupsController::validateGroupEmail via a closure: https://laravel.com/docs/6.x/validation#using-closures

I think that would be faster indeed as the entitlements join seems unnecessary for the uniqueness constraint.

Maybe add an index for the name field for the uniqueness checks?
Maybe add a uniqueness contraint for name + email (so the uniqueness check in the role only additionally checks that you don't reuse the same name for any of your domains)?

To fully validate a group name I need a domain. And we get the domain from an email address. So, I first validate the email address, and only if it is valid I do the "full validation". If email address is invalid I still want to do a "basic validation" on the name.

It will not use an index, though. That's why I hesitate.

Name alone cannot be unique. An index on name + email does not really make sense to me. Maybe this table should have a domain or domain_id column, but it shouldn't be a stopper.

Makes sense. It seems I have overlooked the array_merge since you're not actually overwriting $errors, so this should work.

FWIW, the alternative I suggested wouldn't look much better anyways:

$email = $request->input('email');
$members = $request->input('members');
$errors = [];

list(, $domainName) = explode('@', $email);
$rules = [
    'email' => [
        function ($attribute, $value, $fail) {
            if ($error = GroupsController::validateGroupEmail($value, $owner)) {
                $fail($error);
            }
        },
    ]
    'name'  => [
        'bail',
        'required',
        'string',
        //Validate the email for the $domainName below
        function ($attribute, $value, $fail) {
            if ($error = GroupsController::validateGroupEmail($email, $owner)) {
                $fail($error);
            }
        },
        new GroupName($owner, $domainName),
    ]
];

$v = Validator::make($request->all(), $rules);

if ($v->fails()) {
    $errors = $v->errors()->toArray();
}

The index on name would not be for uniqueness, but for lookup performance.

The name + domain index would indeed be what we're after.

I agree this is not a blocker.