To start a chat with someone, you first need to add them to your contacts.
After sending a contact request, wait for the other person to accept it. Once they do that, both you and them can send each other direct messages and invite each other to group chats.
If the person you want to chat with doesn't have a Status profile yet, you can invite them to join Status.
To add someone to your contact list, you need their chat key, which represents their public Status profile. They can simply send it to you, so ask them to share their Status profile with you. You can also use their QR code to add them to contacts.