Hi Michael,
To use the VS community version, we need to sign in using the Microsoft account by design using the network, it is by design.
Other community members also asked this kind of issue before, maybe you could vote this feature request:
https://visualstudio.uservoice.com/forums/121579-visual-studio-2015/suggestions/9563619-enable-vs-community-activation-registration-offlin
Best Regards,
Jack
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