The SSO login method setup window shows a Default User Group setting when added to a site. There is also a UserGroup option available on the dropdown menu towards the bottom of the window under Fields Mapping.
SSO login will only assign a user to one site group on login. If a mapping is defined for UserGroup, then as long as Pressero has a Site Group that matches to that name, the user will be assigned to that one site group. The SSO users can only be assigned to one site group when logging into the site from the identity provider system.