I’ve signed up for a GROUP conversation, so, I believed, it should have costed 500 points for 60 minutes.
But that’s what I’ve found in my points transaction list:
Conversation attendance charge for15 minutes at 2012-03-24 19:00:00
The length of a group conversation depends on how many participants there are. If there are three students, it will be 45 minutes, as each user pays for 15 minutes. Hope this clarifies things!