If one or more of your contacts belong to more than one group, you may receive an error. Please follow these steps to resolve the issue:
- Go into your phone's address book.
- Find the contact listed in the error message.
- Go to the next contact and remove that contact from all groups except one.
- Go back and try WhatsApp again.
- If it gives the same error, but with a different contact this time, repeat the steps above until WhatsApp works.
To learn how to add contacts, read this article.