The developer program overview docs (https://developer.microsoft.com/de-de/games/xbox/docs/xboxlive/xbox-live/developer-program-overview) say, that Multiplayer is NOT supported in the Creators Program, but is this only related to the Online-Multiplayer and I'm still able to login multiple users, so they can play together / against each other?
Yes, multiple local users is supported. We're adding multiple local user support to the Xbox Live Unity asset in an upcoming release.
Here's the link to Windows Store policies: https://msdn.microsoft.com/en-us/library/windows/apps/dn764944.aspx#pol_10_13