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