By default, Telegram syncs your contacts to its servers. When a new contact joins, you get a notification about it. Your contact will also come to know that you're using Telegram. If you want to keep your identity private, you can stop the Contact Sync feature.

How do I stop Telegram from notifying contacts?

Open Telegram and tap “Settings,” which is located in the bottom-right corner next to Chats. Then, select “Notifications and Sounds.” Scroll to the bottom and toggle off the “New Contacts” option. Once you do this, Telegram will no longer send you notifications when people join.

Why do I get notification when someone joins Telegram?

You might see contact join notifications in Telegram from random contacts. That happens because you let Telegram access your phone contacts. Basically, those 'random contacts' are actually saved in your contacts list and now you might have forgotten them.