Telegram is a good messenger platform and relatively secure. Run out of Dubai by Russians. Not open source. Does let you choose end to end encryption.
Groups
Telegrams best feature is its group handling and range of groups on offer. For many users this is the main reason to use this messenger system. But a major drawback of the system is the default Telegram settings cause it to automatically download and retain a large number of media files, eventually causing the phone to become unusable. This behaviour can be corrected, below.
Preventing and expiring downloads
If you are a member of Telegram groups, it necessary to apply these settings to prevent Telegram making your phone unusable through excessive downloads. It is not uncommon for a member of a Telegram group to unthinkingly post a video a quarter of a gigabyte in size and most people are initially aware of the settings needed to mitigate this.
Doing these things greatly enhances Telegram usability and is worth telling all users to do, especially the less technical and new users. You can show them this page.