When a user opens your map-enabled SharePoint page, they automatically view the map as a guest until they sign in to an ArcGIS named user account.
Guest users have limited access to the ArcGIS Maps web part. In addition to viewing publicly shared SharePoint and ArcGIS content that has been added to the map, guest users can pan and zoom the map, turn layers on and off, and view pop-ups. However, most functionality is restricted to viewing only. Private layers do not display in guest mode and tools that consume credits are not available. To fully interact with the map, users must sign in to ArcGIS using a named user account.