For my visa application I need an invitation from a relative/acquaintance living in Germany. What form does such an invitation have to take?
In the first place, an invitation proves the purpose of the intended journey. Therefore, no special formalities are required. If, however, the host intends to cover the expenses associated with the trip, a formal pledge to cover all costs (declaration of commitment) has to be presented with the visa application. In this case the host can sign the relevant form (“Verpflichtungserklaerung”) at his/her residency's local authorities (usually Ordnungsamt or Auslaenderbehoerde).