Pin or unpin a chat
Open the chat menu on any chat (one-to-one or group).
Toggle Pin Chat on.
The chat moves straight to the top of your chat list with a pin icon next to it. You can pin as many chats as you like, and they'll all sit at the top.
To unpin, open the chat menu again and toggle Pin Chat off. The chat returns to its normal position based on the most recent message.
Note: Pinning is personal to you. It doesn't change anything about the chat for anyone else in it.
FAQ
If I pin a chat and then archive it, does it stay pinned?
Yes. Pin and archive are independent. If you unarchive it later, it reappears pinned at the top.