Thank you for reaching out to us and I'm very sorry for the longer waiting time.
This is what my research has shown: If you want your friend, who is a non-European citizen, to attend your marriage ceremony, they will have to apply for a Schengen Visa for Visitors. Here is a Schengen visa information website with all the required documents your friend will need to gather and where they can apply for it: https://www.schengenvisainfo.com/visiting-friends-relatives/
And more information by the German Federal Foreign Office: https://www.auswaertiges-amt.de/en/einreiseundaufenthalt/visabestimmungen-node#content_1
As you can see, one of the core requirements is a "Letter of Invitation" by the host: in this letter, you have to include the relationship between you two, all the core information (full name, date of birth, etc.) of your friend and the purpose of their stay (the marriage ceremony). In case your friend is staying with you or in a hotel/ accommodation, you need to include this information in your letter as well.
This is what the German Federal Foreign Office says about inviting a friend:
"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)."
Regarding the procedure: as the Schengen Visa website I linked here describes, your friend would need to follow these steps:
"To complete your Schengen Visa application for Visiting Family, Friends or Relatives, follow these steps:
- Check if you qualify for a Schengen Visitor Visa.
- Figure out where you should apply.
- Find out what is the best time to file your visa application.
- Fulfill the Schengen Visa application form.
- Gather the required documents for Schengen Visa for visiting Family or Friends.
- Make a visa interview appointment.
- Attend your visa interview.
- Pay the visa fee.
- Wait for the embassy to process your visa application."
To check whether or not your friend needs a visa to enter Germany, have a look at this overview: https://www.auswaertiges-amt.de/en/einreiseundaufenthalt/-/231148
Please have a look at the linked websites for more information on the duration of the process and when to apply, how long the visa is valid for, and what happens if your visa application has been rejected.
Here is some more information on how to write the Letter of Invitation: https://www.schengenvisainfo.com/invitation-letter-schengen-visa/
If you scroll down, you can find a sample letter as well.
I hope this helps. Do feel free to reach out to the community again if you have any more questions!