No, it means 15,000 new (paying) members of the association Wikimedia Deutschland.
Just to be clear - these are basically donors, not volunteers, right?
However, we also try to recruit new authors/editors via banner campaigns and are running several of them throughout the year. If you are interested in this, consider joining Verena's session at Wikimania about "On-Wiki Campaigning to Engage New Editors"[1]
I look forward to hearing about this but won't be at Wikimania -will there be anything published about it ? :)