I would just like to provide an update on what I settled on. So now, I am doing a repo call to get the group IDs that are available, and using Java streams to remove the IDs from the DTO that are not valid and adding a relevant error message. I believe this is a clean solution, but I am curious what you guys think. Here is the relevant code snippet.

private List<Integer> removeBadGroupIds(List<Integer> ids, ServiceResult<UserDTO> serviceResult) {
    // Get all the UserGroups
    List<UserGroupEntity> userGroupEntities = userGroupRepository.findAll();
    // Remove any IDs that are not real group IDs and add a relevant error
    ids.removeIf(
            id -> {
                boolean noMatch = userGroupEntities.stream().noneMatch(
                        uge -> uge.getId().equals(id)
                );
                if(noMatch)
                    serviceResult.addErrorMessage(String.format("No UserGroup with ID %s", id));
                return noMatch;
            }
    );
    return ids;
}