Send Notification to user using delegated permissions

Abhi1222 - Messages in group chat will be visible to the group members hence you are getting the error. Unfortunately, Microsoft Graph API does not provide a direct way to trigger a notification to a person who is not a member of the group chat. However, you can add a user to the group chat using Graph API. 

Please refer this example to Add a single member to a Microsoft Teams chat, sharing the whole history of the chat - https://learn.microsoft.com/en-us/graph/api/chat-post-members?view=graph-rest-1.0&tabs=http#example-3-add-a-single-member-to-a-microsoft-teams-chat-sharing-the-whole-history-of-the-chat.