If you're the group admin, you can invite people to join a group simply by sharing a link with them. To share a group invite link:
Go to the group in WhatsApp, then tap the subject of the group.
Alternatively, tap and hold the group in the CHATS tab. Then tap Menu > Group info.
Tap Invite via link.
Choose to Send link via WhatsApp, Copy link or Share link via another app.
You can also share a group invite using a QR code by tapping Menu > Print group QR code. To print the QR code, your phone needs to be connected to a printer. Android recipients of the QR code can scan the code using a third-party QR code reader. iPhone recipients can scan the code using their phones' cameras.
The admin can Revoke link at any time to make the previous invite link invalid and create a new link.
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Note: Any WhatsApp user you share an invite link with can join the group, so only use this feature with trusted individuals. It's possible for someone to forward the link to other people. If so, those other people can also join the group and the group admin won't be asked to approve those other people before they join.