To build a group by using WeChat QR code, you need to create a group chat first, then generate a group QR code in the group chat settings, and finally share the QR code with people who need to join the group chat.

First, open WeChat, enter the discovery page, click on the group chat, and then click the+sign in the upper right corner to create a new group chat. In group chat, click the "……" button in the upper right corner to enter the group chat settings page. On the group chat settings page, click "Group QR code" and then click "Save to photo album" to generate a group QR code. Next, share the generated group QR code with people who need to join the group chat, which can be shared through WeChat, QQ, email, etc. When someone scans the QR code, they can automatically join the group chat.