The group is the first radio-focused firm to join the association, which represents both independent podcasters and companies producing podcasts.
The trade body was created in April 2006 to promote podcasting and protect podcasters' interests.
It lobbies the UK government about podcasting issues and fights to ensure people have the right to podcast, rather than having to gain a licence.
GCap Media's podcasting firm Creation produces dozens of podcasts, including material for Capital Radio, Xfm and Classic FM.