To add someone to a WhatsApp group without being an admin, open the desired group chat and click on its name at the top of the screen. Scroll down and select 'Invite Group via Link,' then copy this link and share it with the person you want to invite; they can join directly using this link. If no such option is available, contact the group's admin, provide them with your friend's number or request their addition manually. Always be cautious when sharing invite links as anyone who has access can join.