You could try the guys at
www.bbpixel.com.
They have integrated the users of my vBulletin board into Joomla. Works perfectly. Maybe they can do the same for Photopost usergroups.
FYI: I run a vBulletin board on a Joomla site. The board is in the look and feel of Joomla (skinned by BBPixel). Users are
vB with BBPixel integration. Photopost also uses the
vB usergroups.