Are there groups of airtightness testers?

Yes there are. The best known are probably the German FLiB (www.flib.de) and the British ATTMA (www.attma.org)

Since September 2012, TightVent hosts an airtightness associations committee (TAAC) with representatives of national/local associations or groups including FLiB and ATTMA (tightvent.eu/partners/taac).