helloBill will never ask for your password or full card number, and Bill won't ask for them in the chat. If a message does, or anything feels off, don't act on it — take a screenshot and send it to [email protected] and helloBill will check whether it's genuine.
You will only ever be emailed by helloBill from an address ending in @hellobill.com. Be wary of scams, and always check the sender's email address.