Telegram uses an SMS-enabled phone number for account creation and as your default user identifier. You have the option of creating a username which others can then use to contact you with (without revealing your phone number), but this is an additional step that most users are not going to take the time to do. It also doesn't seem possible to add usernames to your contact list–which seriously limits the utility of usernames.

To start a conversation, you select an entry from your contact list or search for a user by their username (if she has chosen to use one). Searching by phone number doesn't seem to work. Usernames are public, and there doesn't appear to be a way to block incoming messages from specific users. There's no reason to think that you can't send messages to random numbers.

The app has a feature that lets you invite users to Telegram in the form of a list of contacts from your phone's 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 used fails to send SMS to invitees unless Google Hangouts is the default SMS app.) Telegram-enabled contacts appear at the top of the contact list and have a graphic next to the name.

TODO

TODO

TODO