meaning of "ochmoq" is literally "to open" and it is used in informal conversations. equivalent of "to join" in Uzbek is "qo'shilmoq" which sounds natural and also not so formal.

"Contact joined Telegram" - "Biror kontaktingiz Telegram ochganida" (Currently in use). My suggestion is - "Kontaktingizdagilar
Telegramga qo'shilganda". Key word in my version is "qo'shilganda" so it means when someone starts to use Telegram.