Hi @kevingeorget , thank you for your post.
I gave this a go too, in line with Geetha's blog you posted and I found I got the same error and other 400 errors.
I then tested it with the below and eventually it worked in this way:
_api/web/sitegroups/getByName('Alan_s Content Members')/Users?$filter=Email eq 'alan@domain.co.uk'
I added an underscore to the start of the URI and placed my email address inside ' '.
Maybe you could try this alone without intergrating PowerApps for now just to see if you are able to get a return from your HTTP call using the URI above and go from there?
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Thanks, Alan
