Visiting is no problem for citizens of the EU and those from Australia, Canada, US, Israel, Japan, New Zealand and Switzerland. As long as they have a valid passport, they can hang out for three months without a visa. Citizens of other countries will have to provide a letter of invitation and potentially complete a form stating that you’re willing to cover all potential legal and medical bills your friend might incur during their visit. Proof of health insurance covering them for their entire stay is also required. The Ausländerbehörde (registrations office) can provide more details.