I never liked the idea to give a company my credit-card data for what supposed to be a free service. That is one of the reasons why I never joined Skype. Anyway, it looks like that you'd need a Skype Premium account for participating in group video calls.
Just for the record, you don't need to give Skype your credit card.
I've used Skype for free in the past to communicate with students overseas, both video, and audio. I don't know about the group calls. I do know, though, that once you get more than 4 people in the room, it does start to lag a little bit due to the various net connections. Eh, I think it'd still be fun.