Telegram uses an SMS-enabled phone number as your identifier by default. You have the option of creating a username which others can then use to contact you with, but this is an additional step that most users are not going to take the time to do.

To start a conversation, a user must be in your contact list. By default, you have no contacts. The app has a feature that lets you invite users to Telegram in the form of a list of contacts from your contact list. When you ask the app to send an invite, it will send them an SMS directing them to get the app.

• The version of the app I fails to send SMS to invitees unless Google Hangouts is your default SMS app.
• It's unclear at present if the app shows you whether a user in your contact list is Telegram-enabled or whether you need to discover this by attempting a session.