This can be simplified

I'm a little confused by this feedback...

Might be missing something super obvious or wondering if there was a typo and we should simply the following code like this:

setPendingUsersToAdd(pendingUsers => {
  const newPendingUsers = new Set(pendingUsers);
  if (newPendingUsers.has(userID)) {
    newPendingUsers.delete(userID);
    return newPendingUsers;
  }

  newPendingUsers.add(userID);
  return newPendingUsers;
});

cc @ashoat

See here. There's no need to check if the set has the item before deleting it... if it has the item, then deleting it will return true, you will skip adding it. If it doesn't have the item, it won't be deleted, and will return false. Then you will add it.

got it, appreciate the explanation!