TL;DR

If it's a small group chat, submit a report. If it's a group chat with more than 10K members (Telegram Channel), those aren't supported on Beeper.

Missing a Telegram group chat from Beeper?

If the Telegram group chat you're missing from Beeper is a small group chat with your friends, colleagues, or family, hit the Report a Bug on your device. Also, gather comparison screenshots between the Beeper app and the Telegram native app and share them in your Beeper Personal Support chat called Beeper Support - your username.

If the Telegram group chat you're missing from Beeper has more than 10K members (Telegram Channel), those aren't currently supported. Hence, they won't be available in the Beeper app.