1.You can through the REST API below to get the user or group that who can access the dataset by the dataset ID .Enter the dataset ID in the Request URL you will get the result you want .
Admin - Datasets GetDatasetUsersAsAdmin - REST API (Power BI Power BI REST APIs) | Microsoft Docs
“ailsa tao” is my security group ,you can see it in the return value .
2.You cannot through a report to get the users that in security groups. Because we create security group and add users in the groups in Microsoft 365 admin center. If you want see the users in security group , you can sign in the Microsoft 365 admin center and see it in Home => Active teams & groups , with an administrator account .You can through the link below to learn more about users in security group .
3.If you want to know which users accessed the report, you can learn more through these links below .
Hope it will help you !
Community Support Team _ Ailsa Tao