Link to a group in Whatsapp

You must be the group admin in order to have access to the link.
Ask the group admin to share the link with you, and you can then send it to your friend.

From WhatsApp's FAQ:

If you are 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. Tap the subject of the group. Tap Add
participant. Tap Invite to group via link. Choose to share the link
within WhatsApp or to another app.
(Source)

More info on how the link works:

When you press on the link, it will open WhatsApp, and it will give you more info about the group, who the creator is, and who is part of the group. You then have the option of joining the group.

If you open the link in a browser, and press the 'join group' button, it will open WhatsApp and give you the option to actually join.