On iPhone or iPad, it is possible to join a WhatsApp group by asking a group administrator to invite you via a link. If the administrator prompts you manually, you won't need a link.
Steps

Step 1. Ask a group administrator to invite you via a link
An administrator can send you a link on any instant messaging app or on a social network.
- To find the link, the administrator must go to the menu Group Info, Choose Add participants, then select Invite via link. He or she can then copy this link and send it to you.

Step 2. Tap the link
A preview of the group's page will be displayed.

Step 3. Select Join this group
You are now part of the group.