If you want to invite someone from abroad who requires a visa for a short stay, you need a 'Proof of sponsorship or Accommodation Provision'. With this, you declare that you stand surety for any possible costs of the stay.
Any possible debts incurred by the visitor will be recovered from you as well. The guarantee is issued for a maximum of 3 months.
A legalised signature is required for a sponsorship or an accommodation provision. This means that the municipality will check your identity and signature. This is the proof that you really are the person who has to sign the document. You are required to schedule an appointment online (choose Signature, legalisation).
Legalising a signature costs € 11.80.