Group module usage. I'm sure this isn't all applicable to everyone. But basically its designed to create a group for a collection when one is created. When an item is created or updated, it should update the group it belongs to.

groups.php

<?php
use Drupal\group\Entity\Group;


/**
 * Implements hook_node_insert().
 *
 * Alternatively could trigger this with a context reaction.
 */
function asu_permissions_node_insert(NodeInterface $node) {
  if ($node->bundle() == "collection") {
    // Create a new group for the collection.
    $pluginId = 'group_node:' . $node->bundle();
    $col_group = Group::create(['type' => 'collection_group', 'label' => 'Collection ' . $node->id() . ' Group']);
    $col_group->save();
    $col_group->addContent($node, $pluginId);
    $col_group->save();
  }
  elseif ($node->bundle() == 'asu_repository_item') {
    // Add the asu_repository_item to the parent collection's group.
    // We are assumming here one member_of relationship.
    $parent = _get_collection_parent($node);
    if ($parent) {
      $pluginId = 'group_node:' . $node->bundle();
      $group_contents = _get_group($parent);
      if (!empty($group_contents)) {
        foreach ($group_contents as $group_content) {
          /** @var \Drupal\group\Entity\GroupContentInterface $group_content */
          $group = $group_content->getGroup();
          $group->addContent($node, $pluginId);
        }
      }
    }
  }
}


/**
* implemements hook_node_update
*/
function asu_permissions_node_update(NodeInterface $node) {
  if ($node != NULL && $node->bundle() == 'asu_repository_item') {
    $utils = \Drupal::service('islandora.utils');
    if (!$utils->haveFieldsChanged($node, $node->original)) {
      return;
    }
    if ($node->get('field_member_of')->entity && $node->original->get('field_member_of')->entity) {
      if ($node->get('field_member_of')->entity->id() != $node->original->get('field_member_of')->entity->id()) {
        _remove_object_from_parent_group($node, $node->original->get('field_member_of')->entity);
        _add_object_to_parent_group($node);
      }
    }
    elseif (!$node->get('field_member_of')->entity && $node->original->get('field_member_of')->entity) {
      _remove_object_from_parent_group($node, $node->original->get('field_member_of')->entity);
    }
    elseif ($node->get('field_member_of')->entity && !$node->original->get('field_member_of')->entity) {
      _add_object_to_parent_group($node);
    }
  }
}

/**
 * Removes an entity from a collection group.
 */
function _remove_object_from_parent_group($entity, $parent) {
  $group_contents = _get_group($entity);
  if (empty($group_contents)) {
    return;
  }
  foreach ($group_contents as $group_content) {
    /** @var \Drupal\group\Entity\GroupContentInterface $group_content */
    $group = $group_content->getGroup();
    if ($group->label() == 'Collection ' . $parent->id() . ' Group') {
      $group_content->delete();
    }
  }
}

/**
 * Add object to group.
 */
function _add_object_to_parent_group($node) {
  $pluginId = 'group_node:' . $node->bundle();
  $group_contents = _get_group($node->get('field_member_of')->entity);

  // Check if the node is already in the group.
  $node_group_contents = _get_group($node);
  if (!empty($node_group_contents)) {
    foreach ($node_group_contents as $gc) {
      $group = $gc->getGroup();
      if ($group->label() == 'Collection ' . $node->get('field_member_of')->entity->id() . ' Group') {
        return;
      }
    }
  }

  if (!empty($group_contents)) {
    foreach ($group_contents as $group_content) {
      /** @var \Drupal\group\Entity\GroupContentInterface $group_content */
      $group = $group_content->getGroup();
      $group->addContent($node, $pluginId);
    }
  }
}

/**
 * Gets the groups that an entity belongs to.
 */
function _get_group($entity) {
  $group_contents = \Drupal::entityTypeManager()
    ->getStorage('group_content')
    ->loadByEntity($entity);
  return $group_contents;
}

/**
 * Gets the collection parent of an entity.
 */
function _get_collection_parent($entity) {
  $parent = $entity->field_member_of->entity;
  if ($parent) {
    if ($parent->bundle() != "collection") {
      $parent = _get_collection_parent($parent);
    }
    return $parent;
  }
}