async verifyInvitation (code) {
    logger.debug('Checking invitation code', code)
    const result = await db('invitations').where('code', code).select('useBy').first()

    if (result) {
      if (result.useBy === null) {
        return true
      }
    }

    return false
  }
Verify invitation code

@param {String} code invitation code

@returns {Boolean} invitation code status