I keep getting notifications when a new user in my contacts joins telegram. Is there a way to disable this?
Short answer is: no!
No way to really disable Contact Joined notifications with Telegram in iOS.
But, if you prevent Telegram from having access to your contacts, it will stop the notifications, but it's not without side effects, like not knowing who sent you a message, so, I don't think it's a viable option.
Regardless if any app doesn’t have granular settings to let you choose which notifications get sent, you can always stop them all.
Open the settings app.
Scroll down to the app in question and disable notifications.
If you need an app to have granular control of notifications, open a bug report with the vendor of the app or see if an alternate app can work with the service.
Developers and services can tell which accounts have stopped getting notifications so they can react before they lose you as a customer.
Simply use Telegram Web
And bingo! you won't be seeing these irritating user joined notifications :)
Here telegram says you need to delete your contacts and then you will not get any notifications for new users joining.
If you prefer to have your contacts synced then you could do what @PavelAlexeev suggested but the chats will still show on your feed, you just will not get a push notification for them. In this case I suggest you use Telegrams relatively new folder feature and that way your feed will not be cluttered.
I personally removed the syncing and I just add the contacts manually to Telegram when someone writes me there, and they called themselves @fdjsnvdljkv with no profile picture...