According to Android Police, they have spotted in the latest version of the beta for WhatsApp the ability to share group links. What this means that when you give the link to someone and they click on it, they will be added to the group chat, assuming that they are clicking on the link via their mobile device and that they have WhatsApp installed.
As you can see in the screenshot, WhatsApp has warned users that posting the link means you’re letting anyone join the group, so if you were to post it on Twitter and you have a ton of followers, don’t be surprised to find some random people pop by. However it will come useful if you’re organizing an event, like on Facebook, and you post the link on Facebook so that people can add themselves to the list.
The screenshot above also shows that users can revoke the link, meaning that if you want to stop people from joining the group, you can revoke the link so that it no longer links to the group chat. This is in the beta version of WhatsApp which means that we should be able to expect it in the next update.