The best way to invite members to use your mobile directory is to use the Import Contacts feature. This allows you to upload an email list via CSV then automatically send email invites to your entire email list. People are automatically connected to your list after creating their account, and if they forget they'll receive 2 gentle reminders.
Prior to using the Import Contacts feature, sending an intro email is a great way to get members excited about the directory and let them know what's coming. Attached is an html template you can import into MailChimp or Constant Contact. You'll need to customize the name of your organization and city.