@christosjubile, You can list all the chats along with the members of each chat using below graph API request. 

GET https://graph.microsoft.com/beta/users/8b081ef6-4792-4def-b2c9-c363a1bf41d5/chats?$expand=members

 Currently this request supports delegated level permissions and it doesn't support for application level permission. Please follow this documentation 

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