Open portal.office.com and go to the site that you want to check
Click on the cog – and go to Site Settings -> People and Groups
click the group you want and in the url will be a group ID.
Copy this into the url in place of the X below
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Open excel -> Data Tab
Get Data from Other sources -> from ODataFeed
Url : https://*****Client****.sharepoint.com/_api/Web/SiteGroups/GetByID(X)/Users