How do I create a session in REST API for a group conversation?

You only create sessions in the Javascript SDK in the browser environment, there is no need to create a session in the REST API.

If you really want to create a session in the REST API, you need to call:PUT api.talkjs.com/v1{appId}/conversations/{conversationId} 

Click on the link below for more information.talkjs.com/docs/REST_API/Reference/Conversations.html