Each contact has a Chat - start a conversation action that can be used to start a conversation.
You can also start a conversation with multiple contacts by choosing Chat - start a conversation from the actions drop down after any search.
In each case, you will choose the convesation type that you want to have, and the chat service that you want to use for the conversation.
Starting conversations based on keywords
Chatbot can start conversations automatically when certain phrases are heard. This functionality is not currently exposed via the UI but you can access it via the
Starting conversations based on other conditions
For more complex scenarios, such as starting a conversation when someone registers for an event, or makes a donation, Chatbot provides a CiviRules action Start a conversation. To enable this, you will need to install CiviRules extension and the Chatbot CiviRules extension.