Germany is part of the Schengen treaty for free travel among its member states (see http://www.auswaertiges-amt.de/DE/EinreiseUndAufenthalt/03_Visabestimmungen/Visabestimmungen.html); beyond this, Germany does not require visa for all Western and many Eastern European countries.
Moreover, Germany has extensive visa waiver agreements with many other countries, including Argentina, Brazil, Australia, Canada, Chile, Hong Kong (SAR passports), Israel, Japan, South Korea, Malaysia, Mexico, New Zealand, Singapore, Taiwan, and the United States, among many others. For the other countries (e.g., China, India), visa processing is usually smooth and with a predictable (positive) outcome.
For visa requirements in full detail see the following link: http://www.auswaertiges-amt.de/DE/EinreiseUndAufenthalt/03_Visabestimmungen/StaatenlisteVisumpflicht.html.
Lessons learned from other conferences, e.g., ACM MobiHoc 2016, tell that little visa issues arise for travelers attending a conference in Germany. There are no “blacklisted” countries. The only problems reported in the past were arising from people working or studying in the US and were not able to travel because they might run into problems returning to the US.
Visa support letters can be issued to attendees as well as authors with accepted papers, posters, or members of the conference committee. Those requesting a letter should allow 7-10 business days to receive it, as requests are handled in the order they are received. The deadline for receiving requests for support letters is the same deadline for early registration.
Please send all visa letter requests with the following information to email@example.com:
Past experience shows that this works well for applying for visas to Germany. In case you specifically need an invitation from the local organizers, please contact the general chairs by email once you have received the letter and include a scan of the ACM letter. We will then provide you with a complementary invitation from the German organizers for the German Embassy or Consulate. We expect that this will usually not be necessary.
ACM does not provide letters for transport of vendor or presenter equipment.