I'm using Flow to create sub sites using a site template. Each site needs individual permissions, so the site is set up with a SharePoint Http request using a parameter to not inherit permissions. That means that there is no default site Owners, Members or Visitors group. That also means that members cannot share the site with others. This can be seen in the Access Request Settings dialog.
Manually, you can go to the page _layouts/15/permsetup.aspx and use the interface to set up new or existing groups for these three roles. But I need to do this with Flow. I use a SharePoint HTTP request to create a new group and I can get the group's ID.