public Task<PortalGroupUsers> GetGroupUsersAsync()
This Method obtains a PortalGroupUsers object which provides user information for a single PortalGroup such as: the usernames of the administrators of the group, and the usernames of the members who can access information in the group. PortalGroup objects are collections (aka. groups) of items, usually related to a specific area of interest such as maps, applications, and tools. Groups allow members to contribute items as a useful way to collaborate.
You do not have to be a logged in user to get information from this method. You can gain this information from anonymous access. However, to obtain information about a group that has a PortalAccess level of Private or Shared you need to use the AuthenticationManager by supplying the appropriate username/password credentials. As AGOL and ArcGIS Portal require the use of long term tokens to access secured information (i.e. information that is Private/Shared).