Currently it is not possible to assign roles to a custom user/user group content type due to a hardcoded user/user_group content type in UserChoiceType (1) and UserGroupChoiceType (2).

Perhaps the setting ezsettings.default.user_content_type_identifier and ezsettings.default.user_group_content_type_identifier should be used?

Ideally the widget for selecting users/groups should be UDW instead of a select. The current interface scales badly with 50+ users in terms of how hard it is to locate the correct user/group. Filed a separate issue regarding this: EZP-30022

1): https://github.com/ezsystems/ezplatform-admin-ui/blob/2a14ad6e5dea3f504c15673f2b381430e2bc23f6/src/lib/Form/Type/UserChoiceType.php#L67

2): https://github.com/ezsystems/ezplatform-admin-ui/blob/2a14ad6e5dea3f504c15673f2b381430e2bc23f6/src/lib/Form/Type/UserGroupChoiceType.php#L67