Complex sentence: He joined the Cologne Progressives, the art group that Seiwert had founded, and soon followed fellow member Gerd Arntz to Vienna to work for the Gesellschafts-und Wirtschaftsmuseum.
Split into simpler sentences: He joined the Cologne Progressives, the art group that Seiwert had founded. AUgustin followed fellow member Gerd Arntz to Vienna to work for the Gesellschafts-und Wirtschaftsmuseum.