Hey Angela!
Thank your for your question! If you want to let someone stay in your flat/house for a few days or longer, you can register on ****://***.fluechtlinge-willkommen.de/.
You can also get in contact with some Facebook groups, such as :
https://***.facebook.***/RefugeesWellcome/timeline,
https://***.facebook.***/groups/freeyourstuff,
https://***.facebook.***/groups/moabithilft
Or some organization, such as :
https://ohlauerinfopoint.wordpress.***/
Best,
Caro