I got a reply from Josh, who wants to set up a studbook for Pelusios and Pelomedusa. I quote it in full here:
I had forgotten about the TMGs. You could volunteer to head to H.grandis management group. The previous coordinator was a member of the old grandis-forum at yahoo-groups.Marnix,
Glad to hear from you. I am still setting up the database with little interest from the TSA as Pelusios & Pelomedusa are category 4 (very little concern). I do feel that the better now than latter approach will pay off. Dan would want to contact Bill Zeigler from TSA as they are looking for a volunteer to head that taxon management group (studbook).
The link below will take them to the TSA coordinator page and from there they can find more information with the plans in place here in the states.