static async validateUserIds (userIdList) {
    // don't even bother searching for users if in anonymous mode
    if (Auth.isAnonymousMode()) {
      return { validUsers: [], invalidUsers: [] };
    }

    try {
      const users = await User.searchUsers({});
      let usersList = [];
      usersList = users.users.map((user) => {
        return user.userId;
      });

      const validUsers = [];
      const invalidUsers = [];
      for (const user of userIdList) {
        usersList.indexOf(user) > -1 ? validUsers.push(user) : invalidUsers.push(user);
      }

      return { validUsers, invalidUsers };
    } catch (err) {
      console.log('ERROR -', err);
      throw new Error('Unable to validate userIds');
    }
  }
Determines whether each user in the list of users is valid or invalid.
@param {Array} userIdList - Array of userIds
@param {boolean} anonymousMode - Whether the app is running in anonymous mode (no users)
@returns {Object} - An object containing two lists, one of valid users and one of invalid users.
                    A valid user can be found in the user's db based on userId