Hi @Anonymous
If you are looking to access SharePoint On-Premises you would need an "on-premises data gateway"
More information here - https://docs.microsoft.com/en-us/flow/gateway-reference
Regards "if they are trying to build the flow to a site they are only a member would this be an issues". It would depend on what the flow does. If it were reading, it may be ok but your permissions on any SP site, library or list in the most would need to be in line with the request from Flow.
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Thanks, Alan